Leibniz and the Proof of God’s Existence from Eternal Truths

Abstract

The aim of this article is to show how and in what stages G. W. Leibniz came to develop his proof of God’s existence from eternal truths. It begins by tracing the main stages in the history of this proof, initially forged by Augustine of Hippo, and discusses the reasons for its renewal in the last quarter of the 17th century in the post-Cartesian context. The article then shows why, despite skepticism towards this proof in his youth, Leibniz finally produced a version of it that he presents in the Monadology even before the ontological proof. My hypothesis is that the development of the proof from eternal truths is directly linked to Leibniz’s reflection on the nature of the possible, a reflection that is itself to be placed in the context of the polemic between Nicolas Malebranche and Simon Foucher after the publication of the Search after Truth. The Leibnizian proof is original in that it rests on the consideration of the reality of possibles, insofar as God is the reason for them, and on the claimed subordination of logic to ontology.

“These eternal truths are the fixed and immutable point on which everything turns”.1

1. Introduction

Lloyd Strickland has rightly shown that early modernity constitutes “a golden age for devising proofs of God’s existence”.2 He has also underlined the importance of these proofs in the constitution of the philosophical systems of this time. Inspired by Augustine, the Leibnizian proof of God’s existence based on eternal truths has received little attention from commentators. They have focused more on his critique and reform of the ontological proof of Anselm and Descartes, as well as his proof from the contingency of the world.3 It is true that the proof from eternal truths is not one of the five proofs of God announced in Leibniz’s 1668–1669 project for Catholic Demonstrations (Conspectus demonstrationum catholicarum). The first is based on the principle of reason, the second on continuous creation, the third on motion, the fourth on the consistency of bodies, and the fifth on moral certainty derived from the beauty of the world. However, at the end of Leibniz’s life, in the Monadology and Principles of Nature and Grace (1714), other proofs were put forward. The proof based on eternal truths (Monadology §§ 43–44) now occupies a major place alongside proofs based on the possibility of God,4 on contingency,5 on pre-established harmony,6 and on the laws of motion.7

One cannot help but be struck by the gap between the program of the Mainz period and the proofs developed in Leibniz’s late writings. The purpose of this article is to attempt to explain the reasons for the emergence of the proof from eternal truths, which Leibniz had not yet adopted in his youth. My hypothesis is that its emergence is directly linked to Leibniz’s reflection on the nature of the possible—a reflection that is itself to be placed in the context of the polemic between Malebranche and Simon Foucher following the publication of Search after Truth. To better grasp the originality of the German philosopher’s version of an argument originally forged by Augustine, I will begin by setting out the history of the proof from eternal truths up to the middle of the 17th century before considering the causes of its reactivation in the last quarter of the 17th century in a post-Cartesian context.

2. Proof of God from Eternal Truths: Invention, Eclipse, and Reactivation

2.1. Invention and Reception of the Proof: From Augustine to Valerianus Magnus

Augustine’s proof, as developed in On Free Will, is elaborated in a dialog between Augustine and Evodius, according to the conditions laid down by the two interlocutors. They agree that the existence of God is demonstrated from the moment the existence of a reality that is higher than our mind is proven, a reality that nothing yet surpasses, that is eternal and unchanging.8 This without the aid of any bodily sense, but by reason alone. The stages of the demonstration can be summarized as follows:

1. There are sensible objects that several people can perceive at the same time (by sight, by hearing), even though each person perceives them through his own senses, which are the same for each person and which are neither altered nor modified by the perception that each person has of them. Similarly, there are intellectual objects which are seen in common by all those who reason, although each one grasps them by his own reason, and which are the same for everyone and which are neither altered nor modified by the contemplation they are subjected to—whereas what we eat and drink is transformed—and, consequently, which remain immutable and whole, whether we think about them or not.9

2. The relations and the truth of numbers are such intellectual objects. They are indeed “at the disposal of all who reason, so that every calculator, each by his own reason and intelligence, strives to grasp this truth; one succeeds more easily, another with more difficulty; another does not succeed at all; nevertheless this truth is equally at the disposal of those who are able to grasp it; and when one perceives it, it does not change or transform like the food of the one who perceives it: and, when we are mistaken about it, it is not itself ruined; but, while it remains true and whole, we sink all the more into error the less we see it”.10

3. The truth of numbers, numbers themselves, their relations, and the laws that regulate them (addition, subtraction, division) are not known by the bodily senses. While I cannot know the duration of the things I know through the bodily senses (sky, earth, and all the bodies I perceive in them), it is certain that seven and three make ten, not only now, but always: “never, in any way, have seven and three ceased to make ten, or ever will seven and three cease to make ten”.11 Consequently, this truth of numbers, which is common to me and to everyone who reasons, is invariable and incorruptible.

4. Other intellectual objects are identical for all, remain unalterable, available to all and true, even though each person perceives them in his own mind and by his own reason. These are all the principles and maxims of wisdom by which we form our judgments: that we should live justly, subordinate the less good to the better, render to each his own, prefer the eternal to the temporal, the incorruptible to the corruptible, and so on.12 The rules of wisdom are therefore as true and unchanging as the rules of numbers—Augustine does not settle the question of what relationship there is between numbers and wisdom, “whether number is in wisdom or comes from wisdom, or whether wisdom comes from number or is in number, or whether it is possible to show that the two names cover the same reality”.13

5. There is an immutable truth that contains all those things that are immutably true. It belongs exclusively to no one in particular but presents itself to all those who see immutable truths “like a light that is miraculously both public and hidden”.14

6. This truth, in whose unity we see all immutable truths, is not inferior or even equal to our mind, but superior to it. Indeed, if the truth were inferior, we could judge it, instead of judging all things, including ourselves, according to it, as we do. If the truth were equal to our mind, it would be mutable like the mind.15 Consequently, the truth “is more valuable than our minds, each of which becomes wise by this one thing and passes judgment, not on it, but on other things through it”.16

7. The mind which, after considering many immutable truths, turns to the truth itself through which all are manifested and made known, unites with it; “holding fast to it as though it were unmindful of the others, it enjoys them all together in the truth itself”.17

8. The conditions required to prove the existence of God are now satisfied. It has in fact been established that there is something superior to our mind and reason: immutable truths, themselves traceable to the truth itself from which they derive. If there is something even higher than truth, that something is God; if there is not, truth itself is God. In both cases, it is demonstrated that God exists.18

Such a demonstration is a posteriori and not a priori like the argument of Anselm and Descartes—called the “ontological” argument since Kant—since it rests on the consideration of our created and finite mind and proves from it that there exists a superior, immutable, and eternal reality. It starts from our relationship with objects, such that this relationship in no way modifies these objects but leaves them intact, always identical to themselves. These objects are not only speculative—numbers, their relations, their laws—but also practical—the rules of wisdom that guide our judgment and action. Experience teaches us that these objects are in their entirety common to all minds and at the disposal of all minds, since what is grasped by the reason of one is identical to what is grasped by the reason of another, and that they remain what they are, whether we think about them or not.

From this, several characteristics can be deduced: such objects are independent of time, place, and the minds that think them; they are not drawn from the senses, but equally accessible, and rightly, to the reason of every rational being; they are immutable in their entirety; they enjoy universal and permanent validity; they constitute the absolute standard by which we judge all things and, as such, the condition of possibility for the exercise of judgment and reasoning and the practice of morality.19 Their superiority implies that our reason is subject to and dependent on them. Finally, they are one and the same in the unique truth from which they derive, in which they participate and through which they are true.20 This unique truth is identified with God himself, at the end of a Neoplatonist-inspired ascent towards the ultimate principle of all things. In our minds, truth appears in many and varied forms, yet they are unified in God, who is their source. Truths are only truths through Truth, just as objects are perceptible through the light of the sun which, by illuminating them, reveals them.21

It should be noted that Augustine speaks of “immutable truths” (incommutabilis) rather than “eternal truths”, and that these truths refer to mathematical truths and laws, to relationships of convenience and perfection between beings (enabling them to be ranked in order of importance), and to moral maxims. The forms or ideas of things do not seem to be included: in any case, they are not explicitly mentioned in On Free Will. However, it cannot be ruled out that they are also included among the eternal truths, if we rely on On the Trinity, in which Augustine declares that what we see in incorruptible truth is not what the mind of each particular man is, but what it must be “by eternal reasons” (sempiternis rationibus).22 And further on: “With the eye of the mind, therefore, we perceive in that eternal truth, from which all temporal things have been made, the form according to which we are, and by which we effect something either in ourselves or in bodies with a true and right reason”.23

Augustine’s texts thus already open up the possibility of extending eternal truths to the essences of things without, however, providing a means of determining the relationship of these essences to existence, i.e., whether they must necessarily refer to existing beings or can just as easily refer to beings that are only possible. Nor is the relationship of these essences (and truths) to the divine nature clearly specified: the question of whether they are uncreated as God is, or whether God is the author of them by his will, remains unanswered. We might think that they are uncreated if they are merged with God himself, but they might not be if God is still superior to them. Indeed, the hypothesis that God is beyond the truths themselves is not excluded. This superiority and the possibility of a divine creation of truths could explain the choice of describing truths as “immutable” rather than “eternal” in order to reserve the latter term for God alone.

My aim is not to retrace the entire history of this proof of God’s existence, nor to study how it has been resumed, modified, and discussed up to modern times. However, I would like to highlight two major milestones in the Middle Ages.

Anselm of Canterbury provides a version of the proof that differs from Augustine’s. However, it clearly relies on a passage from Soliloquies, where Augustine asserts that truth can never cease to be, for if it ceased to be, it would be true that it had ceased to be. Truth is therefore always, and “something true cannot exist if truth does not exist”.24 Anselm, in turn, argues that truth can have neither beginning nor end, for no proposition can be true unless truth endures. Suppose, in fact, that truth could begin to be and that it could end. Before it is, we would have to say that it is true that there is no truth; after it has been, we would have to say that it is true that there is no truth. Consequently, there would be truth before truth begins to be and after it ends. This is contradictory.25 The truth that ensures the truth of any particular proposition and its permanent validity is the supreme truth, which subsists, unique, by itself, and has neither beginning nor end. In it resides every true proposition, and it is identified with God himself.26 In his Questions about Genesis, Marin Mersenne ranks Anselm’s argument among the reasons that prove the existence of God. While Anselm did not explicitly qualify truth as eternal, Mersenne draws from his argument the conclusion that “truth is eternal and is God”, that it is “necessarily from all eternity and by itself”.27

A more obvious reworking of the Augustinian proof set out in On Free Will appears in Bonaventure’s Itinerary of the Mind into God. Bonaventure makes judgment (dijudicatio) a path to eternal truth, for it is based on a reason that is abstracted from place, time, and change, and is therefore “immutable, limitless and endless (immutabile, incircumscriptibile, interminabile)”.28 Now “nothing is absolutely immutable, limitless and endless, unless it is eternal”; and “everything that is eternal is either God or in God”.29 If everything we judge with certainty we judge by this kind of reason, it follows that God is “the reason of all things, the infallible rule and the light of truth, in which all shine infallibly, ineffaceably, indubitably, irrefutably […]”30 The laws by which we judge all sensible things with certainty are always present, immutable, and necessary, and, as Augustine put it, “no one judges of them but by them”. They are also uncreated, “eternally existing in the eternal art”,31 by which and according to which all things are formed. The affirmation of the superiority of the law over the one who judges by it once again allows us to arrive at God, since nothing is above the human mind other than he who made it.32

Bonaventure also takes up the argument that, since truth is immutable while our mind is changeable, our mind can only contemplate truth by means of another light that shines immutably and therefore cannot be created.33 While Augustine was content to speak of the independence of truths from the created mind, since they persist whether he thinks about them or not, Bonaventure seems to go further: he expressly states that the necessary truth of the inferences we produce derives not from the actual existence of things, since that is contingent, nor from their existence in the minds which conceive them, without which these things would only be fictions, but from the “exemplarity in eternal art” in which they are represented.34

In the 17th century, the Capuchin friar Valerianus Magnus, explicitly claiming to follow Augustine and Bonaventure, took up this path to God through the light of the mind in his work On the Light of Minds and Its Image (Antwerp 1643). This light, by which minds understand and judge all things, is identified with eternal reason, immutable and one. It is God and what makes him immediately known to us. Its immediate intellection replaces metaphysics, since this light “defines singular beings, determines eternal truths and deduces immutable conclusions, commands and excites the love of eternal goodness”.35 From this light “eternally emanate the reasons for all existing or possible beings”.36 Its immediate apprehension is therefore the only answer Valerianus Magnus gives to the question of whether God exists, suggesting that the very question of his existence can only be asked because this light is given to us.37 God’s existence is proven because God is the condition of all intelligibility and intellection: it is in his light that we understand, judge, and reason, and we cannot understand and judge anything, nor reason about anything without it.

2.2. Reasons for a Certain Lack of Interest in This Proof

Despite the authority and influence of Anselm and Bonaventure, two major figures of medieval thought, it seems that the proof from eternal truths did not enjoy the same success as other proofs of God’s existence (notably the proofs by motion, causality, contingency, or the proof by world order and finality). These have gained ground among theologians and philosophers to the point of eclipsing it. Indeed, the proof from eternal truths seems to have remained very marginal before the last quarter of the 17th century. Thomas Aquinas did not explicitly include it among the famous “five ways” he distinguished for demonstrating God.38 We could certainly be tempted to link it to the fourth one, which consists of starting from the observation of the qualities possessed by created things in varying degrees and working up to the being in which these qualities are realized in the highest degree. In this case, it is a matter of going from what is more or less true to that in relation to that which is more or less true is said to be so, i.e., that which is supremely true and coincides with that which is supremely being (God). But even if this interpretation were admissible,39 Thomas Aquinas, by not expressly or directly promoting the proof from eternal truths, certainly contributed to its being underrated, if not neglected. It should be noted that on the Protestant side as well, it was not given prominence, since it is cited, for example, neither by Timpler nor by Vorstius nor by Alsted nor by Derodon.40 Nor is it mentioned in the metaphysics manuals of the first half of the 17th century, such as those by Burgersdijk and Heereboord.41 Théophile Raynaud ignores it in his Natural Theology (1622) when he examines the question of God’s existence (Distinction V, question I).42

How can we explain this disaffection with the proof from eternal truths? And how, on the contrary, can we explain the revival of interest in it in the last quarter of the 17th century, with Malebranche, Diroys, Bossuet, Fénelon, Norris and, of course, Leibniz?

Many theologians and philosophers have certainly expressed doubts about the validity of this proof and have consequently been reluctant to adopt it. There are three possible reasons for this.

1. The proof can be criticized for being based on a certain confusion of terms, and for amounting to question-begging. For it does not so much demonstrate the existence of God as assume it. The immutability, eternity, and necessity we find in intellectual truths, the laws of mathematics, the rules of wisdom, and the essences of things are attributes derived from our relationships, as changeable, finite, and contingent beings, to these objects. But these attributes do not have the same meaning when applied to God. Hence, the inevitable equivocation: in the case of truths, laws, rules, and essences, immutable means universally valid, in relation to the becoming of the created world, whatever the circumstances, regardless of time and place. God is immutable insofar as he is the supreme being. He is always identical to himself and cannot change because he is pure act and nothing in him is in potentia. Eternal means that truths, laws, rules, and essences endure in time and never perish whether we think about them or not. But God is not eternal because he subsists through time, relative to the created world, but because he is outside and independent of time (which begins with the creation of the world). Finally, the necessity of truths, laws, rules, and essences does not refer to a necessity in itself, to the necessity of what is absolute and per se, as God is. It refers either to a logical necessity (that the opposite of which is impossible, i.e., contradictory) or, in the case of the essences of things, to a conditional necessity, such that if the existence of such and such a thing is posited, in the world or in the human mind, then that thing must necessarily have such and such characteristics and properties (in accordance with what its nature, considered as possible, comprises). In other words, truths, laws, rules, and essences are not immutable, eternal, and necessary in the way that God is, so that we cannot legitimately conclude from their immutability, eternity, and necessity that God exists.

In fact, their immutability, eternity, and necessity are inferred negatively: they are concluded from their opposite, i.e., as the negation of the change, ephemerality, and contingency that are proper to the created world of which we are members. It is a misnomer to associate them with those of God or to maintain that they require those of God. In reality, however, this association can only be made if we have first established the existence of a God who is himself immutable, eternal, and necessary. We can only assert that these truths, laws, rules, and essences are immutable, eternal, and necessary because we have already assumed that they reside in God, that they merge with his essence, or that God conceives them, and therefore that there is a God. Admittedly, one might object, it is true, as Bossuet would assert,43 that if human understanding did not exist, and even if the world were annihilated, everything that is true would remain true in the divine understanding. But divine understanding is not thereby demonstrated: it is only presupposed, and therefore the existence of God with it. And this existence is precisely what needs to be demonstrated.

2. The proof from eternal truths leads to a consequence that may seem questionable from a theological point of view: namely, that in contemplating these truths, laws, rules, and essences, we contemplate God himself. If God is Truth, in which all particular truths are contained and participate, then the mind that grasps the truth sees the essence of God. Of course, it is always possible to reply that God thinks these truths, laws, rules, and essences without being them, i.e., without being made identical to them. But the problem is, then, to know their exact status. If they are distinct from God, since they are immutable, eternal, and necessary like him, do not they become independent of him? Obviously, it is out of the question to think that they could be superior to him. But their separation and complete independence from God is no less difficult to sustain in the context of a theology that excludes the possibility of a reality outside the one and only God, while possessing the same attributes as he (immutability, eternity, necessity).

Thomas Aquinas relates these passages from Augustine, which suggest that in this life we can see God in his essence, since it is in him that we grasp the truth, and it is through the eternal immutable truth that is in him—and in which resides the reason for all things—that we can know and judge.44 Thomas points out that, in order to see God, a mind must be totally freed from the bodily senses, either by death or by some rapture. As Scripture says: “Man cannot see me and live”.45 Augustine’s words should not be misinterpreted. Thomas believes that the truth in our minds is not the truth as it is in God and as it can be identified with his essence. It is made in his likeness: “what is known by the soul is true in so far as some likeness exists in it of that divine truth which God knows”.46 There are some truths that men unanimously recognize, such as “the first principles of understanding, both speculative and practical”. In this way “an image of divine truth is reflected universally in the minds of all men”.47 So it is fair to say that whatever the mind grasps and judges by these principles, it sees in divine truth, but “as in a mirror”. This is what Augustine meant. In this case, Thomas concludes, the knowledge of God that we can derive from the thought of these truths is no more valuable than the kind of knowledge we derive from the senses: it does not provide a path to God superior to that of cause through its effect. Such a conclusion is a way of admitting that the “proof” of God’s existence from eternal truths in no way constitutes a new and specific path, since it can be reduced to the path through causality.

3. The proof of God from eternal truths can only be valid once the question, disputed by the Scholastics of the “constancy of the subject” (constantia subjecti), has been resolved, i.e., whether a proposition made about a subject can have real truth if that subject does not exist. In other words, whether or not the truth of propositions depends on the actual existence of the things on which they bear. The difficulty lies in reconciling the authority of Augustine, who affirms the immutability of truth with the authority of Aristotle, according to whom the statement will be true or false depending on whether the state of affairs it refers to exists or does not exist.48 Clearly, if we maintain that a proposition is true as long as the subject to which it refers currently persists, truth is not eternal. If, on the contrary, we maintain that truth is independent of the existence of this subject, then it is eternal. What remains to be shown, however, is what grounds it and ensures its persistence, despite the non-existence of the thing declared to be true. While the thought of the thing in a created mind can serve temporarily as a substitute for the subject missing in existence, it is clear that it cannot suffice to make truth eternal. It is then tempting to turn to God, whose intelligence understands all these truths and guarantees their immutability and subsistence. But for this argument to be valid as a proof of God’s existence, it would still have to be shown that: (1) the presence of true propositions in God is not incompatible with the simplicity of divine intellection,49 and (2) that these truths would not persist if, per impossibile, there were no God, contrary to what is argued by the followers of Duns Scotus and by Grotius.50 For if it is true that the three angles of a triangle are equal to two rights and that man is a rational animal, that there is no triangle and no man in nature, that there is no created or even uncreated understanding to think about, then eternal truths are not likely to provide solid proof of God’s existence.

The question of eternal truths and essences thus shifts from theological and apologetic to metaphysical and epistemological terrain. It may be hypothesized that this shift explains the disappearance of the proof from eternal truths in Scholastic disputes devoted specifically to demonstrating the existence of God. Consideration of the status, nature, and modality of these truths and essences is dissociated from consideration of God’s existence, which is established elsewhere by other proofs. The reference to God in the dispute over the “constancy of the subject” in fact presupposes that his existence has already been demonstrated, rather than serving to prove his existence. Proof from eternal truths is therefore no longer proof, or more precisely, cannot be proof: it presupposes the prior resolution of a dispute, which brings God into play and, consequently, establishes his existence.

2.3. A Renewal Prepared by Descartes but Achieved by Malebranche

Given the criticism and reservations surrounding the proof from eternal truths,51 it is surprising to see it suddenly revived after 1670 in French Catholic circles, with Malebranche, Bossuet, Diroys, and Fénelon; in England with Norris; and independently and almost concomitantly with Leibniz. What accounts for this revival?

My hypothesis is that Descartes is the originator of this proof, although he did not defend it himself, but other proofs in the Third and Fifth of the Metaphysical Meditations. In this text, he inaugurated an original approach, which consists of establishing science on certain and indubitable foundations, starting from the thinking subject and the sole consideration of the ideas within him. Among these is the idea of an infinite substance of which he cannot be the author, since his cause must contain at least as much formal reality as this idea contains objective reality (i.e., represented being). Only a truly infinite substance could have put this idea of an infinite substance into me,52 a finite being. This a posteriori (or effects-based) proof of God’s existence is based on the principle of causality. It is obviously different from the proof from eternal truths, but readers of the Meditations will no doubt have noticed that the approach taken is open to imitation. Descartes shows how it is possible to prove God from the ideas I have within me, whose properties are such that I can in no way be the cause of them. A similar line of reasoning is all that is needed to construct the proof from eternal truths: among the ideas I discover in myself are those of mathematical objects, with their laws, but also the rules of practical wisdom. Their immutability, persistence, and necessity cannot be attributed to the subject who thinks them. The latter, by virtue of his mutability, mortality, and contingency, cannot be their author. These truths can therefore only derive their immutability, persistence, and necessity from a subject who is himself immutable, persistent, and necessary, unless they are him, i.e., unless they merge with his essence.

Let me repeat: this proof, as I reconstruct it, is not Cartesian. But its general outline may be inspired by the approach taken by the author of the Meditations in his attempt to demonstrate God through his idea in me, and on the basis of the properties of this idea. Descartes thus played a role, in spite of himself, in the resurgence and subsequent promotion of the proof from eternal truths. After having left the theological terrain, where Augustine had initially placed it, to move on to the metaphysical and epistemological terrain within the framework of the Scholastic dispute over the constantia subjecti, the proof would have taken the opposite path thanks to Descartes, but without his having intended it: it would have returned to the theological plane to serve as a demonstration of God’s existence.

Of course, it could very well be argued that the influence was, in a way, the inverse. It could be argued that it was not the proof from the idea of infinity, as Descartes expounded it in the Meditations, that served as a model for the proof from eternal truths, as it would be expounded after him, but that it was in fact the proof from eternal truths, as Augustine expounded it, that served as a model for Descartes. So, in reading Descartes, one would have been inspired not so much by his approach to the proof from eternal truths as by Augustine’s, which they found renewed in the Meditations, through recourse to the “way of ideas”.

Be that as it may, Descartes’ mediation does not seem questionable. However, it would not have sufficed without another intermediary: Malebranche’s theory of vision in God. According to this doctrine, set out in Search after Truth (1674), we do not see the ideas of things in our soul but in divine reason, so that we can only know anything by virtue of our union with it. Although Malebranche proposed his own demonstration of God’s existence,53 it is clear that this doctrine offers a theoretical framework conducive to the development of a proof from eternal truths. It favors it but does not immediately lead to it, at least not initially. Firstly, because Malebranche makes it clear that, strictly speaking, and contrary to what Augustine states, God is not Truth, and the truths called eternal are not God. This is why we do not see God by seeing them, but by seeing the ideas on which these truths depend. For truths consist of relations, which are nothing real, unlike ideas, which are real.54 Secondly, because, as we know from Malebranche, we do not just know in God the truths that have to do with immutable and incorruptible realities, but also those that have to do with all the sensible, changeable, and perishable things we experience in the world. Indeed, without the vision of God “in some way”, insofar as his essence includes all ideas, “we would see no thing”.55 This is why Malebranche considers that God’s existence can be proved “in a thousand ways”.56 Anything can be used to demonstrate it: we need only contemplate an idea to have proof that God exists. Finally, unlike Augustine, Malebranche considers that moral laws are not seen as such in God, but known indirectly, through the impression that men constantly receive from the divine will, which carries them towards him (the supreme good) and strives to make their will conform to his.57

Malebranche’s initial disagreements with Augustine are set aside in the Tenth Elucidation of Search after Truth in which a version of the proof from eternal truths is explicitly expounded. Simon Foucher’s criticisms of Search after Truth, particularly with regard to the nature of ideas and truths, are directly linked to this evolution. The use of the Augustinian proof is particularly useful in countering the skeptical arguments of the canon of Dijon. Malebranche-like Leibniz, as we shall see, thus took up this approach, or at least drew inspiration from it, in order to safeguard the immutability and necessity of truths, which Foucher challenged, as well as the reality and subsistence of ideas, which Foucher reduced to the fading species of things.

The Critique of the Search after Truth, published by Foucher in 1675, was therefore of major importance in the history of the proof from eternal truths, since it is likely that this text contributed greatly to its revival by those who saw in it a means of responding to the objections of the canon of Dijon. The latter considered that Malebranche assumed the existence of necessary truths without proving it. He recalls the philosophers who refuted it, and, in particular, the ancient Pyrrhonians, who maintained “that there was nothing constant, nor anything truly determined, or that if there was something constant, we could know nothing about it”.58 As for the “new” skeptics, they “remain in agreement that there is nothing immutable about individuals, and that they are subject to continual vicissitudes”.59 Foucher sets aside mathematical truths and those concerning the essence and existence of God, but considers that other truths, concerning physics, medicine, and morality, cannot reside elsewhere than in “the species and essences of these same individuals, who are subject to change”.60 However, we are justified in thinking that these species and essences are reducible to ideas and that their immutability is only apparent.

In Foucher’s eyes, Malebranche has not proved that these so-called necessary truths are immutable by their nature, nor how they could be so if God were their author, unless he intended to make them immutable, which is also not demonstrated.61 Foucher finally shows the fragility of this supposition of eternal truths, as long as it is not established by or on (natural) theology. For he claims that the existence of God and knowledge of his attributes are required for it to be proven that these truths are necessary and immutable. Indeed, supposing that there are such truths in the sciences, that they are necessary by their nature, that their immutability is the effect of God’s free will, that their necessity comes from the fact that his will is not subject to change, and that God has decided to maintain them always immutable, “it would still be necessary to suppose, in order to enter into [Malebranche’s] sentiment, both the science of God’s existence, and that of his will, that of his freedom and that of his power”.62 Foucher himself establishes the dependency of truths on God. But the path he indicates to Malebranche, starting from God to deduce these necessary and immutable truths, is the opposite of that followed by the Oratorian: the latter will not go from the existence of God to eternal truths, but rather from eternal truths to God.

In the Tenth Elucidation, Malebranche expressly places himself under the patronage of Augustine, referring to On Free Will. His demonstration is carried out in four main stages.

1. Malebranche’s starting point is the natural capacity of all men to know the truth, i.e., their participation in “a certain Reason that they do not determine”,63 i.e., of which they are not the authors, which is identical for all and which everyone can consult. Here, we find the characteristics that Augustine had already noted: truth is common to all minds and available to everyone. It is the same for me and for others, but I do not see it in the minds of others, nor do others see it in mine. This Reason that enlightens all intelligences is therefore universal.

2. It is also immutable and necessary. Malebranche relies on Augustine to show this: ideas about things are immutable, and eternal truths and laws (mathematical truths, moral principles) are necessary, because “it is impossible that they should not be as they are”.64 Now I see in myself nothing immutable or necessary, and no mind can see truths and laws different from those I see.

3. Reason is infinite, a property that Augustine did not mention, because of the infinity of its objects, their properties, and their relationships, and consequently, because of the infinity of the truths that can be discovered in it.65

4. If Reason is universal, immutable, necessary, and infinite, it is none other than that of God himself, “for only the infinite and universal being contains in itself an infinite and universal reason”.66 Malebranche further departs from Augustine by stating this doctrinally unorthodox proposition: that this Reason is “in a sense more independent than God himself”, insofar as God is subject to it, necessarily following it when he acts-whereas Reason is subject to nothing and no one. Now God cannot depend on anything outside himself-otherwise he wouldn’t be God. Reason must therefore be indistinguishable from him and “coeternal and consubstantial” with him.67

The affirmation of this quasi-independence of universal Reason from God is doubly polemical: on the one hand, it marks a clear opposition to the Cartesian thesis of the divine creation of eternal truths since it posits that these truths are uncreated and do not derive their immutability and necessity from God or his will.68 On the other hand, it is a way of responding to Foucher by taking the exact opposite position. While Foucher makes the immutability and necessity of truths depend on God’s existence, Malebranche suggests that they would subsist and be necessary even if God did not exist. It follows that the existence of the truths seems better founded than that of God, since it does not need to posit God in order to be demonstrated.

The Oratorian even reverses the relationship of dependence established by Foucher, since it is not the existence of God that makes it possible to demonstrate the immutability and necessity of truths, but the immutability and necessity of truths that make it possible to demonstrate God, if indeed we are dealing here with a perfect demonstration of God and not, in reality, with the postulation of his existence. For the universal, unchanging, necessary, and infinite Reason that is demonstrated is, first of all, distinguished from God-who is thus assumed, under the designation of “universal and necessary being” rather than proved, before, of course, being identified with him and judged “coeternal and consubstantial” with him. In other words, Malebranche proves and proves only that there is such a Reason which he then associates with God, whose existence, strictly speaking, he has not demonstrated, but only postulated.

Let us take the analysis a step further: does Malebranche really prove the immutability and necessity of the truths that constitute Reason? Or does he rather take this immutability and necessity for granted, as a primordial fact, absolutely certain, which therefore requires no demonstration, as a demonstration would only render it more obscure? This is what he replies to Foucher in the preface to the second volume of Search after Truth (1675), declaring: “[…] if 2 times 2 is necessarily equal to 4, if a whole is necessarily greater than its part, there are necessary truths. I don’t know why the author of the Critique [Foucher] wants me to think of proving what can only be proved by something more obscure or less clear”.69

2.4. Diroys and Bossuet

The resurgence of “the” proof from eternal truths, whatever its flaws and shortcomings, occurred in the post-Cartesian context and was a reaction to Foucher’s skepticism, at least in Malebranche and, as we shall see, in Leibniz. Before turning to Leibniz, let me say a few more words about the development of this proof in the French Catholic milieu of the second half of the 17th century. A study of its fortunes among Leibniz’s contemporaries will show that this proof of God’s existence, while obviously neither the only one available nor the only one put forward, was, so to speak, “in the air of the times”, and yet that, despite undeniable commonalities, it presented differences from one author to the next. This study will help us to better understand the originality of Leibniz’s own version of it, in the context of his own metaphysics.

The version of the proof from eternal truths provided by François Diroys, in Proofs and Prejudices in favor of the Christian Religion, against false Religions and Atheism (1683), is undoubtedly the least original, as it is the closest to Malebranche’s own. The demonstration can be summed up as follows:70 (1) “There are propositions which are eternally and immutably true”, in the fields of logic, morality and mathematics; (2) These truths “are always the same, independently of places and times”; (3) To know something eternal and unchanging is to conceive of God; (4) Consequently, God exists.

Diroys next raises an objection that he may have found in Foucher: that demonstration would be defective, since the object to which these true propositions refer is a creature, for example, a body. But the latter is mutable and has no relation to divine nature. Diroys’ answer is reminiscent of Malebranche’s response to Foucher: we must distinguish between the object of truth (a temporal, mutable creature and its actions) and the qualities of this truth (which governs the creature and its actions, and which is eternal and unchanging).71 Finally, the form of “Platonism” professed by Diroys is clearly inspired by the Oratorian. The truths concerning creatures refer to the eternal and unchanging ideas upon which they are made, which are in God,72 and from which they are indistinguishable. Consequently, “these truths do not enable us to know what God is in all his extent, but they do show us the relation of God to creatures and of creatures to God […]”73 As Malebranche argued, to see these ideas in God is not to see God, strictly speaking, except insofar as the ideas of finite creatures participate in his infinite essence.

This demonstration, which is less developed than the one set out in the Tenth Elucidation of the Search after Truth, nevertheless has the same flaw: it proves the existence of eternal truths rather than the existence of God, because it fails, or at least succeeds only artificially, in showing that the former necessarily entails the latter. God, then, is not so much proved as presupposed. Let us recall Diroys’ main argument: if creatures are in time and subject to change, the eternity and immutability we observe in the truths that concern them, i.e., in their ideas, must relate to a being that itself escapes time and change, and is therefore eternal and immutable. This being is God, and therefore God exists. The flaw in the “proof” lies in the unjustified shift from the recognition of eternal, unchanging ideas to the recognition of an eternal, unchanging being in which they are supposed to reside. It lies in this way of linking “Platonism” (though compatible with paganism) to a dogma of Christian theology (the existence of the one God, creator of the universe), concluding from the former to the latter without showing the validity of this inference. For, if the existence of eternal, unchanging ideas were perfectly established, which Foucher would contest, it would still be necessary to prove that these ideas require an eternal, unchanging being because they derive from it and express, albeit imperfectly, its infinite essence. One cannot go from these ideas to God without first having shown that these eternal archetypes merge into one and the same “exemplar” or “original”, which includes them all, and that this “exemplar” or “original” is identical to God unless, once again, one already has the idea of God, i.e., has postulated it beforehand.

The interest of the proof as conceived by Bossuet and Leibniz, each in his own way, lies in the fact that it makes it possible to correct this shortcoming, overcoming the hiatus between “Platonism” and Christian theology, albeit at the cost of redefining the nature of ideas and their relationship to God.

Bossuet sets out his version of the proof in the Treatise on the Knowledge of God and of Oneself, which he probably wrote between 1677 and 1678 for the Grand Dauphin (Louis of France) for whom he was tutor. The text was only published posthumously, first anonymously in 1722,74 then in 1741 with the author’s name. Most of Bossuet’s contemporaries, with the notable exception of Fénelon, ignored it. In his Treatise, Bossuet makes no direct reference to the polemic between Malebranche and Foucher. Although it is unlikely that he would have read the Tenth Elucidation at the time of writing, it is certain that he was aware of Search after Truth and the controversy that had been raging since 1675 between Malebranche and Foucher. We can therefore assume that the demonstration he proposes in chapter IV (Section 5) is a way of taking a stand against Foucher without completely siding with Malebranche. For although it undeniably shares common features with Malebranche’s proof, his demonstration differs from it and can be interpreted as another possible response to Foucher’s skeptical arguments.

In Bossuet’s presentation, we find several characteristic elements of proof already established by previous authors from Augustine onwards:75 (1) The rules of proportion, the demonstrations produced in the sciences, mathematical objects and properties, and moral duties have in common that they are eternal (i.e., independent of all time) and invariable; (2) All men can see these truths, and they are identical for all; (3) We are not their authors76 because we see them in a light superior to our own. It is this light that enlightens us, and it is through this light that we understand and judge; (4) All these truths are one, and this one truth is God himself.

But new elements are introduced. Bossuet not only asserts, like Augustine, that these truths are independent of us and subsist whether we think about them or not, he also declares: (1) That these truths (and their understanding) do not depend on the existence or non-existence, in nature, of the things they concern, since it is enough to have the idea of these things in the mind to know that “no matter whether they actually exist or not, they must be this way and it is impossible that they be of another nature or happen in another way”;77 (2) That these truths would not cease to be truths, even if all nature were annihilated except me, who would preserve them in my thought, but also “even if I myself were destroyed, and even if there was no one capable of understanding them”;78 (3) That, far from concluding that they are independent, or quasi-independent of God, as Malebranche maintains, we must seek the subject in which “they subsist, eternal and immutable as they are”; (4) That this subject is “a being in which truth is eternally subsistent and in which truth is always understood “,79 that this subject is truth and all truth, that it is God, “eternally subsistent, eternally true, eternally truth itself”.80

Bossuet does not conceive that ideas and truths could exist by themselves and not depend on a subject in which they reside. This is because he does not understand ideas in the manner of Malebranche, but in accordance with Descartes’ teaching, as modes or forms of thought, which cannot therefore exist without it. The very nature of ideas and truths is to be understood by a mind. The requirement of a subject who thinks of them makes it possible to avoid a simple Platonism, which would amount to positing an autonomous locus of ideas and truths whose existence would not necessarily require God, and which, consequently, would lead to supposing the existence of God instead of actually demonstrating it. This is a way of correcting one of the weaknesses Foucher identified in Malebranche’s reasoning.

The heart of the argument can be summed up as follows: “it must necessarily be the case that the truth be somewhere very perfectly understood”.81 Truth cannot be eternal and immutable, as it is, unless it is eternally, fully, and immutably understood as truth and as the same truth. This can only happen if there is a being who eternally, fully, and immutably understands it as truth and as the same truth. This being can only be itself eternal and unchanging. This being is God. Thus, we must recognize that what we understand to be eternal and unchanging we understand in and through God, because God thinks it fully and always, so that what we understand is what God himself understands.

3. Leibniz’s Reinvention of the Proof of God from Eternal Truths

3.1. Leibniz’s Initial Reservation

Leibniz did not meet Bossuet during his stay in Paris (1672–1676), so his own proof of God’s existence from eternal truths was developed completely independently, without any possible knowledge of the French prelate’s writings on the subject. Yet it was conceived at virtually the same time. Is this near-simultaneity a mere coincidence? It may be interpreted as the effect of a reaction shared by Bossuet and Leibniz (after Malebranche’s own reaction) to Foucher’s skeptical arguments, although this reaction took a specific form for each author. Bossuet does not mention Foucher directly, but he writes in the context of the polemic between the latter and Malebranche, of which he was well aware. As for Leibniz, it is clear that, like the Oratorian, Foucher played a key role in the genesis of the proof as he formulated it, the first version of which became available in 1677.

Prior to his 1675 letter to Foucher, Leibniz’s writings contain no trace of the proof from eternal truths. It does not figure among the demonstrations of God’s existence announced in the draft of Catholic Demonstrations (1668–1669).82 A chapter is even planned to denounce the “paralogisms” of which some have been guilty on the subject, including, in addition to Descartes, Valerianus Magnus. As we have seen, the latter is one of the modern restorers and promoters of the proof from eternal truths in On the Light of Minds. Leibniz certainly takes a very dim view of the reasons the Capuchin gives in this book for the existence of God. In his letter to Oldenburg dated 28 December 1675, he refers to the fragility of the arguments of those who claim to derive the existence of God from “the notion of this Being whose essence is to exist”, including the Scholastics, Valerianus Magnus and Descartes, as long as it has not been shown that this Being is possible and intelligible.83 This negative judgment is confirmed in a later text, at a time when he will have elaborated his own proof from eternal truths, in which Leibniz declares that “the opinion of those who want us to see all things in God, that God is the agent intellect, the light of minds”, and the “philosophy of Valerianus [Magnus]”, are “logical paradoxes”.84

The young Leibniz believed in proofs that he considers more solid. These include proof from the principle that “nothing is without reason”, and proof from physics.85 However, the proof from the principle of reason provides the framework within which to prepare the conceptual elements that will be used to constitute the proof from eternal truths-so that it can be said that, at least in the early 1670s, the proof from eternal truths is related to the proof from the principle of reason, or is in some way derived from it. This is particularly apparent in the letter to Wedderkopf (May 1671) and in the Confession of a Philosopher (1673).

Prior to these two texts, Leibniz was already arguing that the essences of things are eternal only because they are in God,86 and that they are not the work of the divine will any more than theorems or propositions that are “of eternal truth”, i.e., that are not “by virtue of God’s will (arbitrium) but by his nature”.87 This independence is not only asserted in relation to his will, it is also asserted in relation to experience,88 as well as to the actual existence of objects in the world corresponding to these essences and truths. In other words, propositions are no less true if the objects they refer to do not exist, or even if there is no world.89 This recognizes the hypothetical character of these abstract propositions (and therefore of eternal truths); to say “if a donkey flies, it has wings” is neither to affirm directly nor to deny that a donkey flies.

God is not the author, by his will, of the essences of things any more than he is the author, by his will, of mathematical truths. These essences, Leibniz declares to Wedderkopf, “contain the very possibility of beings, which God does not bring about, as he does their existence, since these very possibilities, i.e., the ideas of things, coincide rather with God himself”.90 Is this coincidence or a relation of inclusion (from parts to whole)? From this time onwards, God’s relationship to essences appears ambiguous: either we must understand that these essences merge with or are part of God’s own essence, or we must understand that they are merely the object of his understanding.91 It is the latter interpretation that the philosopher of Confession of a Philosopher seems to defend when he asserts that the ideas of things subsist from all eternity in the divine understanding. However, Leibniz makes the following clarification:

“This is to say that God did not make these things [the ideas of nine, the square and all things] by willing them, but by understanding them, and he understood them by existing. For if there were no God, all things would simply be impossible, the number nine and the square along with everything else. You see, then, that there are things of which God is the cause not by his will but by his existence”.

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This passage calls for three comments:

1. God is not demonstrated—indeed, this is not Leibniz’s aim here—but rather assumed. God’s existence is the cause of essences and truths, and it is not the case, conversely, that essences and truths are the reason for God’s existence. Yet an argument in favor of his existence, based on the consideration of essences and truths, is suggested, if not sketched out, without being completed: “if there were no God, all things would simply be impossible”. Pushing the argument ad absurdum to its logical conclusion, we might conclude: There are possible things; therefore, God exists.

2. Already in this text, Leibniz establishes the necessity of linking essences and truths to the existence of God. This link implies that we must not only posit God for things to exist (i.e., for there to be a world, a square, a man, etc.), but also posit his existence for these things to be possible. God’s existence is the sine qua non condition of the possible itself, not just the sine qua non condition of its actual realization, i.e., of the possibility of its creation. In other words, as Leibniz would later argue,93 without God there would be not only nothing existing, but also nothing possible.

3. If essences and truths are contained in the divine understanding,94 it is in the sense that God thinks them by being, that he is the cause of them through his intellection, by “understanding” them. These essences and truths do not seem to precede their intellection but seem to be concomitant with it and to occur at the same time as it by virtue of God’s existence. By existing, or rather because he exists, God immediately conceives them. Chronologically, then, there is no primacy of the object over its intellection. Is there one, from a logical point of view? No, if we consider that essences and truths exist through God’s understanding, since they exist because and insofar as he understands them. And he understands them because he is. But what exactly does he grasp through this intellectual act? His own essence, if it is true that the ideas of things “coincide” with it, as the letter to Wedderkopf teaches.

But this letter teaches us something else too. Leibniz starts from a truth of fact—Pilate’s damnation—and shows that the ascent from reason to reason, in application of the principle of reason, leads inevitably to God, to his will. He then asks: “What, therefore, is the ultimate reason of the divine intellect? The harmony of things. What is the ultimate reason of the harmony of things? There is none”.95 Since divine understanding has a reason, and this reason is the harmony of things—i.e., the ideas of things—beyond which there is no higher reason, there is indeed something that is logically (though not chronologically) prior to the act of intellection, which is precisely what this act of intellection is about, and which it therefore presupposes. Divine understanding, in this sense, is the act by which God thinks of himself and grasps his being, taking himself as the object of reflection. It is from this reflection that all ideas of things are born. Now, if these ideas—essences, but also truths, proportions, and intelligible relationships—are the effects of this reflection insofar as they reflect and express the divine essence, the question arises as to whether they are endowed with a reality of their own, or whether they are reduced to pure mental representations, partial and limited, of this infinite essence, which they duplicate at the same time as they fragment it, with no ontological consistency to speak of.

Leibniz does not settle the question. However, his conception of the possible before 1675, because it is based on this model of cogitation for thinking about the divine production of ideas of things, invites us to see in these ideas (which include the possibles) objects that, formally, are deprived of their own distinct reality if we separate them from what they are the ideas of (the divine essence). As I have shown,96 the metaphysics that preceded the 1675 letter to Foucher is dominated by a conception of the possible that I have called “weak”, which consists of defining it by exclusively logical and epistemological characteristics (non-contradiction, clarity, distinctness, intelligibility). In On the Omnipotence and Omniscience of God and the Freedom of Man (1670–1671?), Leibniz writes: “what is possible is that which can be clearly defined, without confusion or internal contradiction”;97 and in another text from the same period: “everything that is clearly and distinctly perceived is possible”.98 The possible is not defined as a being or as a reality in itself, since it is always related to a subject who thinks of it and, as such, considered a simple modality of his thought: “the possible is what can be conceived, that is (in order that the word can not occur in the definition of possible), what is conceived clearly by an attentive mind”.99

There is nothing to suggest that these definitions apply only to human minds; on the contrary, there is every reason to believe that they apply to every thinking mind. If this is the case, then when Leibniz asserts that essences and truths are in God, he means that they are in his understanding, and that what they refer to (their object) merges with his being. In this sense, their reality is God’s reality, and their existence coincides with God’s existence. But when he considers these essences and truths in themselves, as the fruit of an intellection in and through which God understands himself, they are no more than forms or modalities of his thought, which no more constitute true beings than in the case of human thoughts. The difference lies only in the fact that the ideas in our thoughts last only as long as we think about them, and fade away when we stop thinking about them, whereas divine ideas subsist eternally. However, the latter do not subsist eternally by virtue of a specific nature, but for the sole reason that God eternally continues to think them. For they do not differ formally from human conceptions, except of course in degree of perfection, in their clarity, distinctness, intelligibility, extension and so on.

What I have called the “strong” conception of the possible is the result of a double operation. On the one hand, it involves abandoning the characterization of the possible in terms of clarity and distinctness, i.e., discarding the epistemological approach, while retaining the logical characterization in terms of non-contradiction. On the other hand, it involves granting it a positive ontological status, by elevating it to the rank of a true thing (res), which is founded in the necessarily existing Being, God. In this way, the possible no longer merely designates the conceivable and intelligible but becomes a reality in its own right. The letter to Foucher marks this turning point. However, it is only a first step, to be followed by a second, from the years 1677–1678, when Leibniz draws out the consequences of this “ontologization” of the possible by endowing it with a propensity and even a tendency and a claim to existence, proportional to the quantity of essence or degree of perfection it contains.100

3.2. Opposition to Foucher

On reading Critique of The Search after Truth, Leibniz did not react in the same way as Malebranche, as shown by his annotations to the text and his letter to Foucher. Firstly, he did not consider Foucher’s statement that “the truths of mathematics are founded only on pure suppositions, which do not instruct us as to what is real outside ourselves”,101 called into question their validity insofar as their hypothetical character, which Leibniz acknowledges in no way prevents them from being true. For while they do not assure us that there is indeed something outside us, they do teach us “what would happen if anything existed there”.102 Their truth cannot be challenged, since it persists whether or not there is something outside us. This independence makes it possible to safeguard “arithmetic, geometry, and a large number of propositions in metaphysics, physics, and morals”.103 This independence is not only with regard to the existence of things, but also with regard to man and his thought, since mathematical truths always remain true whether we think about them or not, “with regard to a man who sleeps as well as a man who is awake”.104 What is more, although these truths do not allow us to rule with certainty on the existence or non-existence of external objects, they do allow us to reason correctly about appearances-if there were nothing but appearances to successfully predict them and even to make them happen as we wish.105 In other words, in the situation where there is nothing real outside me, in a world made up solely of appearances, their validity and usefulness would be no less: they would always be confirmed.

On the other hand, Leibniz contests Foucher’s assertion that these truths are neither demonstrative nor immutable, since they depend on “abstractions that our mind takes the liberty of making at its discretion”, so that our mind is “the author of what is fixed in these truths”.106 Leibniz writes the following comment in the margin to signal his disagreement: “never”. Foucher’s position is that the mind substitutes for reality abstractions that it has forged itself, and on which it then reasons: lines, figures, extension, movements, numbers. If we do away with these abstractions, the so-called truths based on them immediately lose all demonstrative and immutable character, and even all necessity. Far from being realities, they are reduced to “mere appearances that are subject to all kinds of uncertainties”.107 It follows then, according to the Canon of Dijon, that “mathematical truths are not truths in the strict sense of the word; or at least they are not the truths that philosophers should be seeking”.108

Foucher’s criticism is twofold: mathematical truths are arbitrary because they are man-made and devoid of any reality, both in themselves (formally) and in relation to the things on which they are based (objectively), which are no more than abstractions. If this is the case, these “truths” lose all validity and necessity and can no longer be distinguished from figments of the imagination, fictions, and chimeras.

In his letter, Leibniz responds to both these criticisms. First, he shows that not only mathematical truths but also those found in metaphysics, physics, and morals are not produced by our minds or the fruit of our freedom because they do not depend on us. They do not depend on us, since the possibility, impossibility, or necessity that true propositions express are not of our making, they are not our work, nor are they chimeras that we arbitrarily invent. On the contrary, they impose themselves on us as soon as we think about them, “since all that we do consists in recognizing them, in spite of ourselves and in a constant manner”.109 They are therefore not derived, but primary in relation to the mind that understands them. The mind has no influence over them. It is impossible, for example, to represent the circle in any other way than with the properties that necessarily belong to it.

Through this independence from the mind of essences and eternal truths, Leibniz concludes that they exist outside the mind. This autonomous existence explains not only their subsistence when we are not currently thinking about them, but also their identity for every mind that thinks about them, i.e., the fact that every mind understands them in the same way, hence their universal validity.

“Thus the nature of the circle with its properties is something which exists and is eternal, that is, there is some constant cause outside of us which makes everyone who thinks carefully about a circle discover the same thing”.

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So, there is indeed something—a reality—that exists outside us. This is what Leibniz wanted to demonstrate to Foucher, who doubted it. But this proof does not consist in demonstrating that the objects we experience in the world actually exist and are not mere appearances. Whether there really are such objects or not, the propositions about them remain true. This is also what Leibniz wanted to demonstrate to Foucher, who contested their validity, immutability, and necessity.

Leibniz’s reference to the “constant cause outside of us”, which means that anyone who thinks about the circle will see exactly the same properties, does not lead to God, as it does for Malebranche. The proof from eternal truths is not developed by Leibniz, as might be expected. Its absence is all the more surprising given that all we had to do was ask: where do these essences and truths reside, if it is true that they subsist, identical for all, immutable and even eternal, outside our minds? Then, the proof would naturally have been introduced. But Leibniz goes no further. He certainly feels that he has fulfilled the goal he set himself once he has proved to Foucher, first of all, that the essences of possible things and the necessary truths refer to a reality,111 existing independently of the mind, its will and its thought, and of the actual existence of objects in the corresponding world; and, second of all, that this reality is permanent, i.e., eternal.

Inscribing possibilities and truths in being, rather than reducing them to a mere mode of thought, is nonetheless a crucial step. By reading Foucher, Leibniz realized that the only way to escape his skepticism and safeguard the validity of eternal truths, necessary in the sciences, was to move away from the epistemological, even psychological approach that Descartes had inaugurated and that was still his. This approach was tantamount to turning truths, ideas, possibilities, and essences into mental abstractions. The risk was that these abstractions, pure intellectual representations, would be assimilated to fictions and chimeras. Foucher did not hesitate to do just that. The metaphysical approach, which succeeds the epistemological, even psychological one, overcomes this difficulty and provides a foundation for the sciences by granting truths, ideas, possibilities, and essences an independent reality and an existence in their own right. The perhaps unexpected effect of this theoretical turn is that it opens the way to a proof of God’s existence that Leibniz had hitherto neglected: “the” proof from eternal truths. Once this “something existing and eternal” outside our minds, independent of the very existence of the world, has been established, the question of its location and relationship to God cannot fail to arise. The existence of a place of Ideas or Forms in the Platonic tradition can certainly be accepted. But in the Christian context, it cannot remain outside of God: it must necessarily dwell there.

3.3. The First Explicit Formulations of the Proof (1677)

It was not until August 1677 that the proof from eternal truths was set out in two contemporaneous texts.112 The reasoning initially takes a short form, which can be reconstructed as follows: (1) Propositions exist and can be known even if the terms of these propositions do not exist; (2) The truth and possibility of the propositions would remain even if all things were annihilated; (3) Consequently, it is impossible for nothing to exist since at least the truth and possibility of propositions exist; (4) The same thing can be proved from the premise that something exists; (5) For, if something exists, necessarily its cause exists; (6) So possibilities, truths, and propositions also exist; (7) “But all seem to exist in one being, which contains ideas”. Hence, the conclusion: (8) “This proof of a necessary and ideal being is not to be despised”.113 For the first time, Leibniz refers explicitly and in positive terms to proof from eternal truths, a proof he had rejected in project for Catholic Demonstrations.

This proof is still linked, through the consideration of causality, to the proof from the principle of reason. It is not “to be despised”, but it does need to be clarified and completed. The rest of the text provides important additions and developments, in particular to justify the transition from (6) to (7): (i) Leibniz shows that what applies to truths also applies to essences or natures: “necessary truths follow from natures”,114 and as truths are eternal, so are natures; (ii) He shows the link between truths, and, consequently, the connection between natures: truths are joined to one another and thereby produce new truths, as do natures, which by joining together form others;115 (iii) Finally, he shows the necessity of positing a subject because these “natures and truths are modes (modi)”.116 From these three premises, he establishes: (iv) That “the cause by virtue of which a necessary proposition is true, while no one is thinking about it must be in some subject a parte rei”;117 (v) That “the cause by virtue of which a proposition about the circle and the square is true is not only in the nature of the circle nor only in the nature of the square”, nor only in the nature of everything that is part of the nature of the circle and the square; (vi) That this cause must be unique by virtue of the unity of the object (of truths and natures) and be found in “something” (implied: in something real). He concludes: (vii) That the nature of the circle, the nature of the square and all the natures that are part of it are “in the subject of ideas, i.e., in God”.118

However, Leibniz does not seem entirely satisfied with this version of the proof from eternal truths, since he gives a new one immediately afterwards. One possible explanation for this rewriting is that the relationship established between the subject and its modes tends to reintroduce the “weak” conception of the possible (and therefore of essences and truths), reducing it to the conceivable non-contradictory, at the risk of reducing it to abstractions and fictions. It is therefore more appropriate to take advantage of the recent achievement of Leibnizian metaphysics, which is the affirmation of the reality of essences and the content of truths, insofar as this affirmation strengthens the proof by means of a novel argument.

Leibniz thus takes up the idea developed in the letter to Foucher, according to which the independence of truth from our thought implies that there is “something real” in it.119 Now,

“since that truth is eternal or necessary, this reality that is in it, independent of our thought, will also be from all eternity. This reality is a certain thing existing in actuality. For this truth always subsists in actuality, objectively (a parte rei). Therefore, there is a necessary being, i.e., in whose essence there is existence”.

120

No reference is made here to God as the being required to eternally think truth, in the manner of Bossuet, in a relationship that is that of the thinking subject to the modes of his thinking. But God is the required being because it is in him, in his essence, that this reality of truth is found, in a relationship that is rather that of an inclusion of the parts in the whole. Truth persists not because God always thinks of it, but because God always is.

The proof is not completely fixed in these texts, however, which are both essays and drafts, where we see Leibniz continuing to elaborate on it. After first linking the proof from eternal truths with the proof from causality, he goes on to show its link with the ontological proof. In a completely new and unexpected way, he derives the ontological proof, as he sees it, from the proof from eternal truths. What is lacking in this proof, which originated with Anselm and Descartes? According to Leibniz, as is well known,121 it is still necessary to show that the notion of being containing all perfections, from which existence is claimed, is possible, i.e., it contains no contradiction. Now, the proof from eternal truths has shown that “a certain necessary being exists”. Since “everything that exists is possible”, we can therefore conclude that this necessary being is possible. The condition set by Leibniz for the validity of ontological proof is thus satisfied by the proof from eternal truths. But it is satisfied in the opposite way to what might have been expected, by moving from the existent to the possible. The proof from eternal truths shows that a necessary being exists in actuality, a parte rei, because the existence in actuality, a parte rei, of necessary and eternal truths requires it. Now, since it exists in actuality, this being is possible. In this case, the ontological argument is not based on proof of God’s possibility, but on prior proof of his existence. It does not demonstrate, as it should, that God exists because he is possible, but that he is possible because he exists, which obviously considerably diminishes its value and relevance.

3.4. The Defects of the Proof and Its Link with the Ontological Proof

The ontological proof and the proof from eternal truths appear intertwined again in January 1678 in a piece of writing intended for Huthmann, when, for the first time to our knowledge, Leibniz places the essences, possibilities, and truths of things in a “region of ideas or truths, or of realities”.122 Clearly, this region is not and cannot yet be identified with divine understanding. Indeed, Leibniz sets out to prove God’s existence on the basis of his essence, i.e., his idea. But this idea cannot be considered in God’s understanding unless his existence is already assumed. To avoid this circularity, the region of ideas and truths constitutes a kind of third term between God on the one hand and the thinking subject and the world on the other. It represents the point of view a parte rei, i.e., truth as it is in itself, taken independently of any mind (including the divine) and of any reality other than itself. It is, as it were, the place where all truths remain eternally. By means of this third term, Leibniz basically intends to establish a demonstration of God outside of God, i.e., without having to assume, in any way, either directly or indirectly, his existence.

Leibniz identifies the possibility of a thing with its possible existence, and judges that this possible existence is inseparable from its essence, insofar as the essence of a thing is the reason for its possibility. It follows the logic that if one of the two (the possibility or the essence of the thing) exists in this region of ideas and truths, then it is proven that the other also exists there. The proof then consists of showing that, because God’s possibility and essence are inseparably linked in the region of ideas, God necessarily exists.123

The interest of this argument, in terms of the history of Leibniz’s development of the proof from eternal truths, lies above all in the introduction of the notion of a region of ideas and truths. The independence of these ideas and truths is no longer limited to God’s will but also to his understanding, insofar as these ideas and truths are not explicitly contained therein. But this is not without its problems. Indeed, this third term (this “region”) is not really a term at all, even if it bears witness to a laudable effort to demonstrate God a parte rei. For the detour via this “place” distinct from God is obviously artificial, since it assumes that something eternal and unchanging is not God himself, and can, by hypothesis, exist outside him: a hypothesis that Leibniz was to abandon in the following decade.

In addition, a second difficulty arises: Leibniz has argued and will continue to argue that eternal truths are hypothetical, if not conditional.124 Now, as he himself notes, for his proof to be valid, eternal truths must not be considered as hypothetical propositions, in the sense that they would presuppose the actual existence of their object.125 For if this existence is presupposed, it is one of two things: either, as Leibniz saw it, the proof is question-begging (God exists because God exists) and therefore worthless, or, as we can easily recognize, the proof is not a proof at all, since the truth on which it is based is a proposition of the type if object x exists, then it will be such and such, so that it simply amounts to stating that if God exists, then he will be such and such (omnipotent, omniscient, etc.). This in no way serves to prove his existence. The truth on which the proof is based is in fact a categorical truth, stating that “a parte rei, while no one thinks about it and in the absence of any condition, it is absolutely and simply true that the essence and existence of God are inseparably connected in that region of essences or ideas”.126 Leibniz seeks to prove God by demonstrating the existence of truths about God, in the independent region of ideas, and not by directly demonstrating the existence of the object of these truths, otherwise he would fall into a circle. Indeed, he considers that it is only from the existence of these truths, once established, that the existence of their object (God) can be concluded. In other words, the point is to prove that God exists because the truth of his existence is founded in the region of eternal truths.

This detour via the third term, as I have called it, has the disadvantage of transforming eternal truths, which Leibniz otherwise declares to be hypothetical, into categorical truths. Certainly, it can be argued, on the basis of a passage in the New Essays on Human Understanding,127 that eternal truths are conditional only with regard to existence but that they are categorical and absolute on the level of possibility, when they state the predicates of the subject under consideration without supposing its existence. But in this case, by stating what the all-perfect Being is, the “proof” once again proves nothing. It does not constitute a specific way of demonstrating God but rather boils down to the ontological argument as it must be reformulated, i.e., as it must demonstrate, from its idea, that the all-perfect Being is possible. Proof from eternal truths achieves nothing if it claims to prove that God exists because it is absolutely and eternally true that he is possible, since proving this possibility is precisely the task of the ontological argument.

In fact, the assimilation of eternal truths to categorical truths is the consequence of the invention of this third term, which posits the existence of truths in themselves, without presupposing the existence of their objects, even hypothetically. It does not allow us to construct an authentic proof from eternal truths. The integration of this third term into God, which will take place in 1686,128 will restore its specificity and relevance.

3.5. A Priori Proof by the Reality of Essences

In the piece of writing intended for Huthmann, Leibniz distinguished God from the “region” of essences and truths, in the hope of achieving a demonstration of God that would not already presuppose his actual existence. The integration into God of this “region” or “land”, which also includes all possibles, was inevitable, but it necessarily leads to a modification of the structure of proof from eternal truths. It definitively separates this proof from the proof from God’s essence or idea and thus establishes its theoretical autonomy. For the proof from eternal truths will no longer consist of showing that among these truths, as they are in themselves, in the “place” assigned to them, there is a proposition which states that it is eternally true that the possible existence and essence of God are indissociably linked, and, therefore, that God exists. It consists of showing that these eternal truths, because they are “something real” and not pure fictions of the mind or chimeras, must necessarily find their “root” or “source” in a being existing in actuality, necessary and eternal. In other words, from being essentially logical in the piece of writing intended for Huthmann, the proof will become or revert to being an essentially metaphysical argument. As in Foucher’s time, this change is explained by the need to reassert the ontological consistency of essences and uncreated possibilities, notably against Antoine Arnauld (in 1686),129 and later against Gabriel Wagner (in 1697),130 who denied them any reality by reducing them to productions of the mind (concepts). But this time, Leibniz goes further than in 1675. He is not content to say that essences, possibles, and truths exist independently of our thoughts and the objects in the world, and, therefore, that there are indeed things that exist outside us, but he affirms that essences, possibles, and truths derive their being from a necessary and eternal being, and can only derive it from him, and that in this way God is proved.

In the version of the proof he delivers, Leibniz is then led to specify (1) the meaning to be given to this reality of essences; (2) the type of relationship, other than strictly causal, that exists between God and these essences; and, finally, (3) how their reality in and through God is to be understood.

(1) The reality of a possible is what makes this possible not just a representation in the mind of something non-contradictory that does not exist. It is what makes the possible, strictly speaking, a thing (res),131 and not nothing, so that it is some being. For if the possible were ontologically nothing, to the extent that “nothing follows from nothing”,132 nothing could be derived from it, i.e., it could not be, and therefore it would not be possible-and nothing would be. Yet, as Elements of True Piety (1677–1678) pointed out, the very fact that there is something rather than nothing implies that there is something in the possible (or essence) “from which actual existence follows”, and, consequently, that the possible involves a “propensity to exist” or a “disposition to exist”.133 The reality of the possible is proved by the actual existence of something, i.e., by an a posteriori argument insofar as this existence can only occur if there is something possible, and if this possible is not nothing, but contains a tendency to exist.134 This tendency to exist, which defines it, is all the stronger, the greater the quantity of essence, i.e., of reality or perfection, that the possible envelops.

(2) This reality, with the claim to existence it contains, can only derive its origin from a being existing in actuality, necessary and eternal, i.e., God, qualified as the “root of possibility”.135 This deduction provides a proof of God’s existence. This proof is now clearly distinguished from the ontological proof, but also from the proof from causality. By insisting on the terms “root”, “source”, and “foundation”, Leibniz emphasizes that God’s relationship to truths is not that of an efficient cause to its effects.136 Just as he is not the cause of essences and truths through his will, as we have known since the earliest texts, neither is God their cause through his understanding. Rather, he is the reason for them, according to the distinction between cause and reason made in the Elements of True Piety:

“For the cause is the reason for the thing outside of the thing, i.e., the reason for the production of the thing; but the reason for the thing can be within the thing itself. And this applies in all things that are necessary, such as mathematical truths, which contain their reason within themselves; likewise God, who alone among actual things is the reason for his own existence”.

137

Just as God is not the cause of himself but the reason for himself, so God is not the cause of essences, of possibles, of truths, but is their reason. He is what makes them be and be real, without having produced them, as if he preceded them or could be conceived outside of them. God’s relationship to essences is therefore not the same as his relationship to existences. He is the source of the former as reason and primary foundation (“there is in God the reality of essences, or of eternal truths”), whereas he is the source of the latter as a productive and even creative cause (“the production of existences, or of contingent truths”138 is in God).

From this point of view, this Leibnizian proof can indeed be linked to the proof from the principle of reason. In addition, it appears less as a proof from eternal truths than as a proof from possibles, even if the difference is minimal, since Leibniz does not consider possibles separately from eternal truths, insofar as eternal truths relate to possibles as they do to all essences. This proof is based not so much, as with other philosophers and theologians, on the immutability, permanence, and necessity of truths, which cannot be created by the human mind, as on the need to recognize their reality and the foundation of this reality in the divine being, unless one reduces them to fictions and denies oneself the possibility of accounting for the actual existence of “something”.

(3) What remains to be determined is exactly what is meant by “there is in God the reality of essences, or of eternal truths”. How are we to understand in what way God is their “foundation” and that they are real in and through him? Essences, possibles, and truths are in God as in their subject. However, they are not there as modes or forms of thought are in the thinking subject, otherwise we would fall back on the “weak conception of the possible”, but because they are realized there:

“[…] it must be the case that eternal truths have their existence in some absolutely or metaphysically necessary subject, that is, in God, through whom these truths, which otherwise would be imaginary, are realized, to use a barbarous but distinctive word”.

139

Essences, possibles, and truths derive their reality from God. They do not exist as the world exists, apart from God, since they are in him. Their existence merges with his. They are realized in God, in the sense that they acquire their reality from him, i.e., they are things, properly speaking through him. This constitution of essences in things through God’s being and existence is recalled again in a letter to Johann Bernoulli in May 1699:

“For the divine essence is, as it were, the region of eternal truths, so that by God’s existence truths concerning non-existent possibles are realized, otherwise they would be deprived of subject and support”.

140

The region of eternal truths indicates a “place”, but this place cannot act as a subject or substrate, since space is not something substantial. Consequently, a real being that contains them is required, a form of Platonism can only subsist for Leibniz, and more generally, in Christian thought, on the condition that Ideas and Forms are located in God. However, it must be emphasized that neither in On the Radical Origin of Things (1697) nor in the letter to Bernoulli, nor earlier in A Specimen of Discoveries of the Admirable Secrets of Nature in General (1688),141 is the divine understanding mentioned in this “realization”, as being its locus and operator. God, his essence and his existence are only indicated as the reason for the reality of essences, possibles, and truths. Yet in other later texts, the active role of divine understanding in the constitution of essences in things is explicitly invoked. Thus, in a letter to Electress Sophie (31 October 1705), “the region of eternal truths” is said to be “realized by the divine understanding”.142 Paragraph 184 of the Theodicy goes on to state that it is “the divine understanding which makes the reality of eternal truths”.143

These passages seem to suggest that the understanding is not simply the specific place in God where truths reside, nor even just the faculty by which God represents essences and ideas to himself, but the faculty that confers reality on them, the faculty that makes them, properly speaking, things. The model of divine intellection as the reason for truths, as put forward in the Confession of a Philosopher, thus reappears. It is closely linked to the epistemological and psychological approach, which makes truths dependent on a thinking subject in order to be and to endure. This approach is clearly echoed in paragraph 189 of the Theodicy: “these very truths are not without an understanding that takes cognizance of them, for they would not subsist if there were not a divine understanding wherein they are realized, so to speak”.144 However, we might have thought that this explanatory model had been superseded by another, the metaphysical model of inclusion (or the relation of parts to the whole), which amounts to integrating possibles, essences and truths into the divine essence itself. We might also have thought that the metaphysical approach to the possible had led to the definitive abandonment of the epistemological and psychological approach. So how should we interpret this resurgence?

The context of the passages quoted from the Theodicy helps us to understand and explain Leibniz’s position. His position lies between two extreme theses that he rejects. The first is that of Descartes, who maintains that eternal truths depend on God’s will, and that God is their author. The second is that of “some Scotists”,145 and also of Grotius,146 according to whom these truths are, on the contrary, entirely independent of God, not only of his will but also of his existence, since they would subsist even if he did not exist. This latter thesis runs the risk of positing an order of necessary and eternal things, distinct from God and even superior to him, since he would be subject to it like all minds. This amounts to subjecting him to a kind of external and inviolable fatum,147 contradictory to his omnipotence and freedom.

For his part, Leibniz maintains that eternal truths (and essences) are independent of God’s will without being independent of his existence. For his understanding is what makes them real, by taking them as the object of his intellection. The notion of realization plays a fundamental role here. It prevents truths from becoming so autonomous that they could exist on their own by marking their dependence on God, who, however, does not create them. At the same time, however, it avoids the kind of dependence suggested by the psychological model: that truths depend on God in the same way as ideas depend on the subject who thinks them and exist only insofar as the subject thinks them. Such a relationship tends to reduce ideas to modes of thought and truths to purely representational content. It is true that truths can only subsist if God continues to think them, but this is because their existence depends and always depends, ontologically speaking, as real beings, on the exercise of his thought. In other words, truths do not subsist so much because they are thought by God (cognitively speaking) as because, by thinking of them, God makes them be (ontologically speaking).

On this point, a certain analogy can be drawn: truths and essences are and endure only insofar as and for as long as God conceives them, so that their subsistence is, so to speak, a continuous realization, just as the world can only continue to exist, once created, if God preserves it in existence through his power, so that the preservation (of the world) is a continuous creation. But there is a big difference here: realization (by understanding) is not the same as creation (by power), otherwise God would be self-caused (the author of his understanding). Despite some perhaps ambiguous formulations,148 Leibniz seems to exclude any relation of exteriority and any temporal relation between the divine understanding and its objects, between the act of understanding and what is understood in it and by it. Clearly, however, this identification of the subject of knowledge (divine understanding) with its object (eternal truths) is problematic for anyone who considers, in the philosophical tradition, that according to the order of things, objects precede the faculty that knows them. Considering that “as soon as truths no longer depend on God, it is God who depends on them”, Jean-Luc Marion argues that Leibniz is led, whatever he says, to re-establish a “relation of objectivity” between essences and divine understanding, to think of “a gap between the object and that which delivers it”, and to admit, in the act of intellection, at least a logical precedence of ideal content over the faculty that grasps it.149

Here again, the notion of realization helps us to understand the original Leibnizian thesis that, in the case of God, the object does not pre-exist the faculty. For, far from grasping an object that is already there, waiting to be thought of, it is the exercise of the understanding that brings the object into being, i.e., gives it its reality. If there is a logical anteriority, it would rather be that of the understanding over truths and, more generally, the nature of things, as suggested by a passage in the discussion with Gabriel Wagner.150 In fact, such anteriority is not even admissible if we consider the numerous passages in which Leibniz declares the understanding to be the region of truths and possibilities. This identity excludes both any priority of understanding over truths, and of truths over understanding. How can we understand this identity? It can be explained by the way Leibniz conceives the faculties (understanding, reason, will): they are inseparable from their objects, or rather they merge with them. For him, the understanding is not a pure power or capacity to know without content. Whether divine or created, it is not mere receptivity, an empty receptacle or a blank tablet waiting to be informed by objects outside of it. It is always already an order and connection of ideas in infinite number. The understanding itself is nothing other than its ideas.151 There is no more pure thought, no more cogito without cogitata, than there is pure will without object or end. There are only particular and varied thoughts (varia a me cogitantur, adds Leibniz to the Cartesian principle),152 and singular and diverse volitions. The relation of truths to the divine understanding is not, strictly speaking, a relation (from subject to object), so that in God the Scholastic distinction between the truth of the thing and the truth of the understanding is erased.153 This rules out any discrepancy or precedence, in favor of reciprocal dependence: essences cannot be without God, nor God without essences, in the sense that God cannot be God without these essences being real.

Before discussing the impact of this doctrine on the proof from eternal truths, two clarifications are in order. First, there remains a major difference between the divine understanding and the human understanding, which threatens to introduce an equivocity in the term “understanding”. While the emergence of truth is exactly contemporaneous with the exercise of the divine understanding (which “realizes” it), the human understanding grasps a truth that exists independently and before it thinks of it. Second, the comparison between understanding and will—there is no such thing as pure thought or pure will—must not obscure another important difference: the one between a divine will that, no more than the human will, cannot produce its own object (volitions), and a divine understanding that, unlike human understanding, produces its object (ideas, truths, essences) through its own activity. Note that the argument used to refute voluntarism in the name of the principle of reason—neither man nor God can will something without reason, nor something that has not been previously provided by the understanding—does not apply in the case of the divine understanding, which does not derive its ideas from elsewhere. Its ideas (and the essence of all things) are as such without reason, that is to say, without it being possible to account for them. From this point of view, and only from this point of view, one must admit a form of arbitrariness in God and a limit to the application of the principle of reason.

What consequences does this doctrine have for proof from eternal truths? It clearly distinguishes this proof from the ontological proof by basing it entirely on consideration of the reality of truths and essences. God exists because truths and essences exist, admittedly according to a way of being that is not that of the objects we experience in the world, but as intelligible and incontestable realities.

The Leibnizian proof of God from the reality of eternal truths can thus be summarized as follows: (1) Truths and essences are real; (2) They cannot derive this reality either from themselves, or from objects existing in the world, without which non-existent possibles would be pure chimeras, or from a created mind, since they subsist whether or not the mind thinks of them; (3) They can only derive their reality from a being existing in actuality; (4) This being is God, because he alone can make them real, instead of merely imaginary or fictitious; (5) So, God exists. From the moment it is entirely founded on the consideration of the reality of truths and essences, this proof, which was a posteriori, i.e., established from effects in the form it took in Augustine, but also in Leibniz’s Elements of True Piety, becomes a priori like the ontological proof, while also being distinct from it.154

3.6. The Primacy of Reality over Possibility

Truths and essences cannot exist without God. Leibniz insists, however, that they derive not only their reality but also their possibility from him: indeed, without God, “there would be nothing real in possibilities, and not only nothing existing, but also nothing possible”.155 Against the Scotists and Grotius, Leibniz asserts the complete dependence of truths and essences on divine understanding. This dependence is first and foremost ontological, with regard to reality and existence, since God must be posited for them to be real and for objects that conform to them to exist in the world. This dependence is also true on the logical level with regard to the definition of modalities (possible, impossible, necessary, contingent), since God must be posited for these essences to be non-contradictory and for the truths of reason to be necessary. Without God, there would be nothing: no truth, no geometry,156 no law,157 no science, and, consequently, no principles of reason and contradiction, which govern being and thought.

This logical dependence on God has been challenged by a number of commentators,158 who have argued that the possible as possible has a natural priority over the reality of the possible. On the basis of the distinction between the possibility of the possible (what formally makes the possible possible) and the reality of the possible (the fact of being an object of divine understanding), Fabrizio Mondadori thus argues the following:

“In fact, possibility is a (necessary) precondition of the reality of the possible: if, at any rate, the quite plausible assumption is made, first, that something has reality (something exists) only if it is intrinsically, or absolutely, or logically possible to begin with; and, second, that the absolutely impossible has no reality of any kind. In a word: being possible is a (necessary) precondition of possible being”.

159

The logical and ontological levels must be rigorously distinguished and, according to Mondadori, the former commands the latter. It follows, conversely, that if a possible is real, i.e., derives its reality from divine understanding, this does not mean that it derives its logical foundation from this same divine understanding:

“From the fact that eternal truths are ontologically grounded in the divine intellect, it need not at all follow that their truth is, therefore, also grounded (modally, that is) in God’s intellect. Were we to contend, in other words, that, if God did not exist, there would be no (such things as) eternal truths, meaning thereby that the reality of eternal truths is grounded (ontologically, that is) in God, we could hardly be taken to be suggesting, let alone implying, that the truth of eternal truths is also grounded (modally, that is) in God”.

160

Consequently, the possibility of the possibles, just as the truth of eternal truths, is absolutely independent of God, from the point of view of what in itself makes the possibility of the former and the truth of the latter. Possibles and truths depend only on the divine understanding for their realization, i.e., for them to be made real. God’s existence is therefore not a necessary condition for establishing eternal truths on the modal plane. Possibility, impossibility, necessity, truth, and essences are what they are even if God did not exist. Clearly, to posit this independence is to ruin the proof from eternal truths.

The letter to Bourguet from December, 1714, seems to support this interpretation, when Leibniz writes:

“I agree that the idea of possibles necessarily presupposes the idea of the existence of a being who could produce the possible. But the idea of possibles does not presuppose the very existence of this being, as you seem to hold, Sir, by adding: if there were no such being, nothing would be possible. For it is sufficient for a being who could produce the thing to be possible, in order for the thing to be possible. Generally speaking, for a being to be possible, it is sufficient that its efficient cause be possible; I make an exception of the supreme efficient cause, which must actually exist. But this belongs to another chapter, that nothing would be possible if the necessary being did not exist. This is because the reality of possibles and of eternal truths must be founded upon something real and existent”.

161

Leibniz himself formulates an objection that could be raised against him. It boils down to this: eternal truths are conditional propositions, such that they state what their subject will necessarily be if it exists, i.e., if it is actually produced. But it is obvious that they presuppose the cause of their subject (so that it may be), as a possibility, at the logical level of the idea, but do not posit it in existence, at the ontological level of what is actual. A necessary being cannot therefore be deduced from a possible being, since a possible being only presupposes, in order to be possible, that another possible being capable of producing it is presupposed. In other words, possible being requires nothing other than the possibility of a being that can cause it. This is why we cannot conclude from its idea that its cause actually exists and is necessary.

Does the objection reach the proof of eternal truths? No, it simply recalls the general case, while excluding God. As far as finite causes are concerned, it is true that from the essence of things we can only conclude the necessity of the possibility of their cause, but not the necessity of the actual existence of their cause. But this does not apply in the case of the “supreme efficient cause”: we must be able to conclude from the essences of things not only the necessity of the possibility of necessary and eternal being, but also the necessity of its effective existence. For the question is in fact not the same, and because it is not the same, the cause at stake is not the same. In one case, we are asking what makes a possible thing actually feasible. The answer is: it is feasible only if its efficient cause is itself possible. The possibility of thing A necessarily contains in its essence the possibility of its cause B, so that if B is posited, A is posited. In the second case, we ask what makes a possible thing real. So, we ask about the reason for the essence of this thing, i.e., what makes it real, and from where does what is real and positive in its content come. The answer is that the reality of the essence of thing A necessarily implies a real and actual cause B, so that, because A is posited, B is posited. Indeed, the answer cannot be that the reality of the essence of a thing A proceeds from a possible being B, unless we ask again where the reality of this second possible being comes from, answering that it is in a third thing C, thus falling into a regression ad infinitum, in which we would never find the foundation of this reality. We must therefore posit “something real and existent” as the ultimate reason. Without this “something real and existent”, i.e., without necessary and eternal being, “nothing would be possible”, i.e., feasible (in the sense of the first question). So, it is not the possibility of the possibles that conditions the reality of the possibles, but the opposite. It is because there is reality that there is possibility (and therefore feasibility), and it is because there is an originally existing being that there is reality. Reality makes possibility, or rather reality and possibility, which are inseparable, depend at the same time on the same intellectual operation of God, which is his understanding.

4. Conclusion

From Augustine to Leibniz, the proof of God’s existence from “eternal truths” was undeniably transformed. It underwent a remarkable resurgence in the last quarter of the 17th century, and my hypothesis is that this resurgence can be explained by the “way of ideas” opened up by Descartes. By endeavoring to prove the existence of things from their ideas, as they exist in the thinking subject, Descartes certainly restored a certain relevance for this proof, which had hitherto been left in the shadows, without however developing it himself. His contribution is, however, ambiguous, for at the same time, he exposed eternal truths to the risk of being reduced to their pure representation in the mind, i.e., to modes or forms of thought. In other words, he gave weapons to skepticism, but also the means to combat it.

Malebranche was truly the one who provided the elements for a rehabilitation of the proof. But the proof would not have been fully completed or formulated without Foucher’s intervention. His skeptical arguments probably led the Oratorian, but also Bossuet and Leibniz, to reinvent it. The years 1677 and 1678 are the period in which, albeit in different and independent ways, these three authors renewed “the” proof from eternal truths, or rather proposed three versions of it. Their common reaction makes this proof the fruit of a form of Augustinianism reactivated in the post-Cartesian context.

Leibniz neglected it in his early texts, no doubt because he considered it logically flawed. Foucher’s reading marked a turning point, the effects of which would be seen in 1677, when the proof was explicitly formulated for the first time. The years that followed would see its gradual autonomy from the proof from causality and from the ontological proof. At the end of this theoretical development, it will have become entirely a priori, and will even be set out (in the Monadology) before the ontological proof.

In contrast to Malebranche and Bossuet, Leibniz’s version is unique. Leibniz differs from Malebranche insofar as he judges that we see things through our own ideas and not in God, where these ideas would be. He certainly sees the immutability and persistence of truths, their universal validity and their independence from individual minds, as an argument in favor of the existence of God, insofar as his understanding is the eternal place where these truths reside. But he probably does not think this is a sufficient argument. He also certainly considers that these truths can only subsist if they reside in a being who always thinks of them (as Bossuet maintains) and who, consequently, must be immutable and eternal like them. But he no doubt thinks that this is not a sufficient argument either, if it leads to these truths being reduced to mere modes of thought, purely relative to the subject who thinks them, with no ontological consistency of their own.

The originality of Leibniz’s proof lies in basing it on the reality of truths and essences. God is not only the being that must be posited for truths and essences to be thought and eternally thought, but for these truths and essences to be truths and essences, i.e., something real and therefore possible. Divine understanding does not grasp truths and essences, which would thus be logically prior to it,162 but realizes them, i.e., makes them real. It is through its activity that they come about, since for Leibniz the faculty is nothing if not a pure abstraction without the objects with which it merges.

The object is contemporary with the faculty that is its reason (rather than its cause). But what, it may be asked, is divine intellection the intellection of? It is the intellection of God himself, in accordance with the well-known Aristotelian thesis that God thinks himself. Confession of a Philosopher already taught us this. Truths and essences come from the fact that God is and understands his essence. God’s intellection and essence are thus distinct, as shown by certain texts in which Leibniz makes eternal truths the expression of God,163 or a representation of what his essence contains: “In the divine essence, things are eminently, [but] in his understanding [there is] something more, namely in a representative way, since in the divine understanding are represented also the imperfections or limits of things”.164 Truths and essences are therefore not, strictly speaking, God, nor are they a kind of reflection or copy of his essence in and through his thinking, since imperfections and limits, which make things particular, are added to them. So, God and God thinking are not exactly the same thing, even if to posit one is to posit the other. Things are in God “eminently” (eminenter), in God thinking “representatively” (repraesentative), with their limits. And they can only be in the latter because they are in the former. Realization by this understanding appears in the passage from the eminent to the representative. A thing is not God, or is no longer exactly God, but his thought. This difference is crucial. Of course, it shows that Leibniz never completely abandons the epistemological or even psychological model of the subject and its modes of thought. But above all, the distinction between the eminent and the representative underlines that the logical plane remains subordinate to the ontological plane, since divine being is necessary for truths, ideas, essences, and possibles to be and to be what they are. They are real only because God is the Ens realissimum.165