The Dynamic Pineal Gland in Text and Paratext: Florentius Schuyl and the Corporeal–Spiritual Connection of the Brain and Soul in the Latin Editions (1662, 1664) of René Descartes’ Treatise on Man

Abstract

The Latin (De Homine, 1662, 1664) and French (L’Homme, 1664) editions of René Descartes’ Treatise on Man present different iconographic traditions, but the iconography of the Latin editions is little known. Dutch physician and botanist Florentius Schuyl edited De Homine and illustrated it himself with a mix of woodcut and copperplate illustrations. This paper examines Schuyl’s innovative depictions of purported dynamic aspects of the pineal gland as claimed by Descartes: (1) repeatedly illustrating the pineal gland as the corporeal–spiritual linkage of the brain and soul; and (2) using a movable flap anatomy to illustrate the pineal gland as a motile structure that both responds to and directs animal spirits. None of the canonical illustrations in the later French edition attempted to depict the corporeal–spiritual linkage of the brain and soul, and the modest attempts in the French edition to depict the motility of the pineal gland relied simply on superimposition of two purported positions of the gland, a technique also employed by Schuyl. This paper also reviews how Schuyl’s illustration of a corporeal–spiritual linkage of the brain and soul in a goat sharply contrasts with his written defense of Descartes’ bête-machine doctrine in the extended preface to De Homine.

1. Introduction

French philosopher René Descartes (1596–1650) [Figure 1] wrote his Treatise on Man between 1629 and 1633, left it unpublished (fearing similar repercussions to Galileo, after Galileo’s condemnation by the Roman Inquisition), and died more than a decade before his work was finally published in competing Latin and French editions with separate sets of illustrations in the early 1660s. The Latin (De Homine; Des Cartes 1662; Descartes 1664a) and French (L’Homme; Descartes 1664b) editions present quite different iconographic traditions, but the iconography of the Latin edition is little known, whereas the iconography of the French edition has predominated since the 17th century.

Dutch physician and botanist Florentius Schuyl (1619–1669)2 [Figure 2] edited De Homine and illustrated it himself with a mixture of woodcut and copperplate illustrations (Des Cartes 1662; Descartes 1664a; Lanska 2024a, 2025). While the contributions of Claude Clerselier (1614–1684), the editor of the later French edition, are usually discussed in reference works on Descartes or his oeuvre, Schuyl is typically overlooked (Cottingham 1992; Descartes 2004; Gaukroger 1995; Gaukroger et al. 2000; Nolan 2016) with few notable exceptions (Descartes 2003).

This paper examines Schuyl’s innovative depictions of purported dynamic aspects of the pineal gland: (1) repeatedly illustrating the pineal gland as the corporeal–spiritual linkage of the brain and soul; and (2) using a movable flap anatomy to illustrate the pineal gland as a motile structure that both responds to and directs animal spirits.3 None of the canonical illustrations in the French edition attempted to depict the corporeal–spiritual linkage of the brain and soul, and the modest attempts to depict the motility of the pineal gland in the French edition relied simply on superimposition of two purported positions of the gland, a technique also employed by Schuyl. This paper also reviews how Schuyl’s illustration of a corporeal–spiritual linkage of the brain and soul in a goat is diametrically opposed to his written defense of Descartes’ bête-machine doctrine in the extended preface to De Homine.

2. Descartes’ Treatise on Man

Descartes’ Treatise on Man was meant to be a section in a larger and all-encompassing masterwork, Le Monde (The World), which was to explain inanimate nature, the human body, and the “rational soul” (Clarke 2003; Descartes 2003, 2004). Descartes had begun writing his Treatise on Man in 1629. In a letter to Mersenne on 5 April 1632, Descartes reported his progress on The World and acknowledged his limitations regarding the corresponding illustrations:

I expect that you have been waiting for me to send you the treatise which I promised you... It is almost finished … but I would prefer to keep it for a few months, to revise it and rewrite it, and to draw some diagrams which are necessary. They are quite a trouble for me, for as you know I am a very poor draughtsman...

(Descartes 1897, p. 242; 1936, p. 220; 1970, p. 22)

Descartes abandoned work on his Treatise on Man (and The World) when he learned of the condemnation of Galileo (1564–1642) following Galileo’s trial for heresy by the Roman Inquisition in 1633. In a letter to Mersenne in April 1634, Descartes wrote:

Doubtless you know that Galileo was recently censured by the Inquisitors of the Faith, and that his views about the movement of the earth were condemned as heretical. I must tell you that all the things I explained in my treatise [The World], which included the doctrine of the movement of the earth, were so interdependent that it is enough to discover that one of them is false to know that all the arguments I was using are unsound. Though I thought they were based on very certain and evident proofs, I would not wish, for anything in the world, to maintain them against the authority of the Church. … I desire to live in peace and to continue the life I have begun under the motto to live well you must live unseen. … I am more happy to be delivered from the fear of my work’s making unwanted acquaintances than I am unhappy at having lost the time and trouble which I spent on its completion

(Descartes 1970, pp. 25–26; see also Descartes 1897, p. 284; 1936, p. 252)

Descartes’ Treatise on Man remained unpublished when he died, but several draft copies of the manuscript were extant, having been sent to colleagues for comment. All of them contained detailed references to figures that Descartes had planned to include in the published version, with very specific explanations referring to specific parts of the figures with letters and numbers. However, apparently none of the circulating copies of the draft manuscript were accompanied by even rough sketches of what Descartes had imagined for these illustrations. Because no version of Descartes’ Treatise on Man was published during his lifetime, and because only a few poor-quality sketches by Descartes were ever found (separate from his circulating manuscripts), Descartes’ disciples were forced to create their own illustrations for his often vague and confusing text.

3. Descartes and the Corporeal–Spiritual Connection of the Pineal Gland and the Rational Soul

Descartes’ views on the location and function of the pineal gland were already present in a primordial form by the 1630s. He made passing mention of this gland in La Dioptrique (Descartes 1637) and briefly discussed the corporeal–spiritual connection of the body and the soul in the fifth part of his Discourse on the Method (1637), although he did not specifically mention the pineal gland there.

Descartes gave a much more elaborate discussion in a letter on 29 January 1640 to Lazare Meyssonnier (1611–1673), physician to King Louis XIII (1601–1643) and professor of surgery in Lyon. Descartes argued that, while most intracranial structures were paired, one on each side, the pineal gland is uniquely solitary, making it the only possible place where the corporeal body could interact with the spiritual soul (even though the pineal gland is not uniquely solitary, since the pituitary gland is also solitary). Descartes also mislocalized the pineal gland to within the ventricular cavity.

I will answer the question you asked me about the function of the little gland called conarion [i.e., the pineal gland4]. My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed. The reason I believe this is that I cannot find any part of the brain, except this, which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and altogether have only one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul. Now it is impossible to find any such place, in the whole head, except this gland; moreover it is situated in the most suitable possible place for this purpose, in the middle of all the concavities [sic, The pineal gland is not located within the ventricular system]; and is supported and surrounded by the little branches of the carotid arteries which bring the spirits into the brain

(Descartes 1970, pp. 69–70; See also Descartes 1899, p. 18; 1947, p. 19)

In a subsequent letter to Mersenne on 1 April 1640, Descartes recapitulated some of what he had written to Meyssonnier concerning the unitary nature of the pineal gland:

I do not altogether deny that the impressions which serve memory may be partly in the gland called conarium [the pineal gland], especially in dumb animals and in people who have a coarse mind. But it seems to me that others would not have the great facility which they have in imagining an infinity of things which they have never seen, if their souls were not joined to some part of the brain capable of receiving all kinds of new impressions, and consequently not at all suitable for preserving old ones. Now there is only this gland to which the soul can be joined; for there is nothing else in the whole head which is not double. But I think that it is the other parts of the brain, especially the interior parts, which most serve memory. I think that all the nerves and muscles can serve it, too, so that a lute player, for instance, has a part of his memory in his hands: for the ease of bending and [moving] his fingers in various ways, which he has acquired by practice, helps him to remember the passages which need these dispositions when they are played

(Descartes 1970, p. 71; see also Descartes 1899, p. 45; 1947, p. 45)

In a letter to Mersenne on 24 December 1640, Descartes explained why he believed the pineal gland, and not the pituitary gland, was the important corporeal–spiritual connection between the material body and the immaterial soul. It is clear from his letter that Descartes still believed in a human rete mirabile (a structure present in ungulates, but not present in humans), believed that the pineal gland is part of the brain while the pituitary is not (in reality, the posterior pituitary is largely a collection of axonal projections from the hypothalamus), and believed that the pineal gland is mobile and localized within a ventricular cavity, not because of empiric evidence, but because his mechanical explanations implied that the place where the soul operates on the body “must be mobile” (emphasis added).

[T]he pituitary gland is akin to the pineal gland in that both are situated between the carotid arteries and on the path which the spirits take in rising from the heart to the brain. But this gives no ground to suppose that the two have the same function; for the pituitary gland is not, like the pineal gland, in the brain, but beneath it and entirely separate,5 in a concavity of the sphenoid bone specially made to take it, and even beneath the dura mater if I remember correctly. Moreover, it is entirely immobile, whereas we experience, when we imagine, that the seat of the common sense … the part of the brain in which the soul performs all its principal operations, must be mobile. It is not surprising that the pituitary gland should be situated as it is between the heart and the conarium, because there are many little arteries there, forming the carotid plexus [i.e., the mythical plexus mirabilis/rete mirabile] also, which come together there without reaching the brain. For it is a general rule throughout the body that there are glands at the meeting points of large numbers of branches of veins or arteries. It is not surprising either that the carotids send many branches to that point; that is necessary to feed the bones and other parts, and … to separate the coarser parts of the blood from the more rarefied parts which alone travel through the straightest branches of the carotids to reach the interior of the brain, where the conarium is located [sic].6

In this letter to Mersenne, Descartes again expanded his explanation for what he considered to be the unique properties of the pineal gland that enabled it to function as the corporeal–spiritual linkage, specifically the supposed location (as misstated by Descartes) of this unpaired structure:

There is no need to suppose that this separation takes place in any but a purely mechanical manner. When reeds and foam are floating on a stream which splits into two branches, the reeds and foam will be seen to go into the branch in which the water flows in a less straight line. The present case is similar. There is good reason for the conarium to be like a gland, because the main function of every gland is to take in the most rarefied parts of the blood which are exhaled by the surrounding vessels, and the function of the conarium is to take in the animal spirits in the same manner. Since it is the only solid part in the whole brain which is single [unpaired], it must necessarily be the seat of the common sense, i.e., of thought [sic, not the traditional concept of common sense,7 but instead an aspect of reason], and consequently of the soul; for one cannot be separate from the other. The only alternative is to say that the soul is not joined immediately to any solid part of the body, but only to the animal spirits which are in its concavities, and which enter it and leave it continually like the water of a river. That would certainly be thought too absurd. Moreover, the conarium is so placed that it is easy to understand how the images which come from the two eyes, or the sounds which enter by the two ears, must come together at the place where it is situated. They could not do this in the concavities, except in the middle one, or in the channel just below the conarium [i.e., the aqueduct of Sylvius]; and this would not do, because these cavities are not distinct from the others in which the images are necessarily double

(Descartes 1970, pp. 85–86; see also 1899, p. 262; 1947, p. 237)

Finally, in a letter to Mersenne on 21 April 1641, Descartes tried to address an appropriate criticism that no nerve terminates (or originates) with the pineal gland. Descartes deflected this criticism by suggesting that the nerves instead terminate on the walls of the ventricle and all of them are connected to the pineal gland through “the spirits”. The appropriate rejoinder to Descartes should have been that no nerve terminates or originates on the ventricular walls, but somehow that obvious point was never raised.

[I disagree with your recent objections about the conarium,] namely, that no nerve goes to the conarium and that it is too mobile to be the seat of the common sense. In fact, these two things tell entirely in my favour. Each nerve is designed for a particular sense or movement, some going to the eyes, others to the ears, arms, and so on. Consequently, if the conarium was specially connected with one in particular, it could be deduced that it was not the seat of the common sense which must be connected to all of them in the same way. The only way in which they can all be connected with it is by means of the spirits, and that is how they are connected with the conarium. It is certain too that the seat of the common sense must be very mobile, to receive all the impressions which come from the senses; but it must also be of such a kind as to be movable only by the spirits which transmit these impressions. Only the conarium fits this description

(Descartes 1970, pp. 100–1; see also Descartes 1899, p. 358; 1947, p. 335)

In Descartes’ Treatise on Man (Des Cartes 1662; Descartes 1664a, 1664b), he wrote of the corporeal–spiritual connection of the pineal gland to the soul in several places, but he was somewhat vague compared to what he wrote in correspondence:

Now I will tell you that when God unites a Reasonable Soul to this machine … he will give it its principal place in the brain, and will make it of such a nature that, according to the various ways in which the entrances of the pores which are on the interior surface of this brain [i.e., the ventricular walls] will open through the intermediary of the nerves, it will have various feelings

(Des Cartes 1662, p. 35; Descartes 1664b, p. 29)

Now among these figures [sensory impressions], it is not those which are imprinted in the organs of the external senses, or on the interior surface of the brain [i.e., the ventricular walls], but only those which are traced in spirits on the surface of the gland H [pineal gland], where the seat of the Imagination and of common sense are located; [the representation of such images on the surface of the pineal gland] must be taken for the ideas, that is to say for the forms or images which the Reasonable Soul will immediately consider, when being united to this machine it imagines or feels some object

(Des Cartes 1662, pp. 81–82; Descartes 1664b, p. 73)

[W]hen there is a Soul in this machine, it will sometimes be able to feel various objects through the same organs, arranged in the same way, and without anything at all changing, except the configuration of gland H

(Des Cartes 1662, p. 93; Descartes 1664b, p. 82)

4. Descartes on the Motility of the Pineal Gland

In his Treatise on Man, Descartes claimed that the motility of the pineal gland is instrumental in distributing animal spirits to appropriate parts of the brain, an opinion in opposition to the second-century views of Galen8 (Galen and May 1968, pp. 419–22; Meyer-Steineg 1911, p. 203; Des Cartes 1662, pp. 87–89; Descartes 1664b, pp. 77–79).

Consider … that gland H [pineal gland] is composed of matter that is very soft, and that it is not entirely joined and united to the substance of the brain, but only attached to small arteries (whose skins are quite loose and pliable) and supported as if in balance by the force of the blood that the heat of the heart pushes towards it; so that it takes very little to determine it to incline and bend more or less, sometimes to one side, sometimes to another, and to make it so that in bending, it arranges the spirits that come out of it to take their course towards certain places of the brain, rather than towards others

(Des Cartes 1662, pp. 87–88; Descartes 1664b, p. 77)

Descartes further opined that an imbalance of spirits flowing from the pineal gland, contributes to pineal movement, like the smoke rising from a chimney, although his analogy is flawed, because rising smoke rapidly becomes chaotic.9

Now there are two principal causes, without counting the force of the Soul, … which can make it move in this way… The first is the difference which is found between the small parts of the Spirits which come out of it: For if all these Spirits were of exactly equal force, and there were no other cause which determined it to flow here or there, they would flow equally into all its pores, and would support it completely straight and immobile in the center of the head… But like a body attached only to a few threads, which would be supported in the air by the force of the smoke which would come out of a furnace, would float incessantly here and there, according as the various parts of this smoke acted against it differently; Thus the small parts of these Spirits, which lift and support this gland, being almost always different in some way, do not fail to agitate it and make it lean sometimes to one side and sometimes to another…

(Des Cartes 1662, p. 88; Descartes 1664b, pp. 77–78)

In any case, Descartes concluded that the spirits emanating from the pineal gland move in a directed manner across the ventricular cavity and are capable of selectively turning the appropriate sets of receptor tubes that he envisioned to be arrayed along the ventricular wall. This entire mechanism was also flawed because the pineal gland is not located within any of the cerebral ventricles as was pointed out initially by Galen in the second century CE and then, shortly after publication of Descartes’ Treatise on Man (1662, 1664), by Danish natural philosopher Niels Stensen (or Steensen; Latinized to Nicolaus Steno, Stenon, or Stenonius; 1638–1686) by 1664 (Galen and May 1968; Stenon 1669; Steno 1965, 2013; Andrault 2016).

Now the principal effect [is] that the Spirits thus coming out more particularly from some places on the surface of this gland, than from others, can have the force to turn the small tubes of the interior surface of the brain in which they are going, towards the places from which they come out, if they do not find them already turned there; and by this means to move the members to which these tubes relate, towards the places to which these places on the surface of the gland H [pineal gland] relate. If we have an idea about moving a member [e.g., a limb], that idea, consisting only in the way in which these Spirits then come out of this gland, is what causes the movement

(Des Cartes 1662, p. 89; Descartes 1664b, pp. 78–79)

For Descartes, the orientation of the pineal gland is sufficient to direct the attention of the soul.

[W]hen there is a Soul in this machine, it will sometimes be able to feel various objects through the intermediary of the same organs, arranged in the same way, and without there being anything at all that changes, except the situation of the gland H [pineal gland]

(Des Cartes 1662, p. 93; Descartes 1664b, p. 82)

In summary, the factors that can contribute to the movement of the pineal gland are the soul, the force of the spirits flowing through it, and impressions from memory.

[W]hen the gland H [pineal gland] is inclined towards one side, by the sole force of the spirits, without the Rational Soul or the external senses contributing to it, the ideas which are formed on its surface do not only proceed from the inequalities which are found between the small part[icle]s of these Spirits, and which cause the difference of the humors…, but they also proceed from the impressions of the memory

(Des Cartes 1662, p. 94; Descartes 1664b, p. 84)

In his letter to Mersenne on 1 April 1640 (seven years after Descartes had stopped writing his Treatise on Man), he expounded his dogmatic belief in the motility of the pineal gland, albeit in the absence of empirical support. Indeed, Descartes dismissed the need for empirical support for his proclamations concerning the mobility of the pineal gland, considering it self-evident, although he acknowledged that it could not likely move through any great displacement:

I need no proof of the motility of this gland apart from its situation; because since it is supported only by the little arteries which surround it, it is certain that very little will suffice to move it. But for all that I do not think that it can go far one way or the other

(Descartes 1970, p. 72; see also Descartes 1899, p. 45; 1947, p. 45)

5. Schuyl’s Illustrations of the Pineal-Soul Connection

Many of the neuroanatomical illustrations in Descartes’ La Dioptrique (1637), De Homine (Des Cartes 1662; Descartes 1664a), and L’Homme (Descartes 1664b) were highly abstract and schematic, whereas some of Schuyl’s copperplate illustrations in De Homine (Des Cartes 1662; Descartes 1664a) sought a higher degree of realism, albeit divorced entirely from Descartes’ text.

A curious feature of Schuyl’s illustrations, one unique to the Cartesian iconography of De Homine (Des Cartes 1662; Descartes 1664a), and one curiously overlooked in Cartesian scholarship, is the incorporation of representations of the corporeal–spiritual connection between the pineal gland and the rational soul that was postulated by Descartes (Wilkin 2003; Chan 2016; Nadler 2016, 2024). This corporeal–spiritual connection between the pineal gland and the rational soul is shown in multiple mid-sagittal diagrams by Schuyl that were meant to support various Cartesian points about the processing of sensory information, the innervation of the viscera by what would later be called the vagus nerve, or the state of the brain in sleep or wakefulness [for example, Figure 3, Figure 4, Figure 5 and Figure 6]. In all these diagrams, there is a billowing fabric-like extension from the pineal gland (universally labelled “H” in the diagrams and in Descartes’ text) that projects upward (superiorly). It appears somewhat jagged in its uppermost extent. This illustrated structure has no actual anatomical correlate, but was simply Schuyl’s attempt to illustrate Descartes’ idea that the pineal gland is the unpaired anatomical structure in the brain that serves to connect the corporeal brain with the incorporeal (or spiritual) rational soul.10

Beyond Schuyl’s Latin editions (Des Cartes 1662; Descartes 1664a) of Descartes’ Treatise on Man, there were no similar representations in the remainder of Descartes’ oeuvre, including Descartes’ only illustrated monograph published during his life with images of the human body, La Dioptrique (1637), nor was any attempt made to illustrate this supposed corporeal–spiritual connection in L’Homme (1664) after Descartes’ death. Even though this feature is present in many of Schuyl’s illustrations in De Homine (Des Cartes 1662; Descartes 1664a), it has apparently escaped recognition in prior discussions of the illustrations in the Treatise on Man, even in those few that discuss the Latin edition (Wilkin 2003; Chan 2016; Nadler 2016, 2024).

Near the end of De Homine (Des Cartes 1662; Descartes 1664a), Schuyl included multiple copperplate engravings of the gross anatomy of the nervous system of goats and humans with much greater detail and seemingly a much greater degree of realism than any other illustrations in either De Homine or in the French edition, L’Homme (Descartes 1664b). Schuyl did not provide any corresponding explanation of these figures. Moreover, none of these clearly corresponded with figures referenced in Descartes’ manuscript, and no similar images were included in L’Homme.

Of the human brain images, one showed the dorsal surface (frontal poles at the bottom and cerebellum at top) with the cerebellum pulled backward to expose the pineal gland (seedlike structure in the center of the image) and the corpora quadrigemina (Des Cartes 1662, Figure 53, Fol. 118) [Figure 7]. The fabric-like material extending from the area of the pineal gland anteriorly in the space between the frontal lobes is not the falx but likely Schuyl’s attempt to incorporate the corporeal–spiritual connection of the pineal gland with the soul into this figure, despite the difficulty of doing so when looking down on the brain from above. That task was more easily represented with the occipital lobes resected, with the seedlike-pineal gland situated at the base between what remains of the two cerebral hemispheres (Des Cartes 1662, Figure 55, Fol. 118) [Figure 8]. Extending upward from the pineal gland is a clear representation of the corporeal–spiritual connection of the pineal gland with the soul. Between the pineal gland and the cerebellum at the bottom are the corpora quadrigemina. Within the remaining portion of the cerebral hemispheres, the only representation of the lateral ventricles in the entire Cartesian oeuvre can be seen.

The remaining two copperplate illustrations related to the human brain in De Homine (Des Cartes 1662; Descartes 1664a) are among the most remarkable attempts of any of his posthumous illustrators to depict Descartes’ ideas concerning the pineal gland. The first of these is a semi-schematic copperplate engraving by Schuyl of a human brain, viewed from above, in which the cerebral hemispheres have been separated and pulled to the side to reveal the quadrigeminal plate with the two enormously and unnaturally enlarged superior colliculi (adjoining round masses) and the pineal gland in the midline just below them (Des Cartes 1662, Figure 54 of Folio 118 and “Nota” following page 121) [Figure 9]. Although generally unrecognized as such, this is actually a “flap anatomy” (i.e., an anatomical illustration with moveable parts), as first noted by Crummer in 1932 (though not illustrated by him and generally ignored afterwards),11 with the pineal gland affixed to an unnaturally large stalk to show its purported mobility, which was postulated as a core feature of Cartesian physiology (shown in the two detail-view strips at right showing just the midline structures from the entire illustration on the left) (Crummer 1932). The tiny flap is engraved on both sides. The stalk itself covers the passageway between the third and fourth ventricles (the aqueduct of Sylvius). This flap-anatomy illustration was present only in the first two editions of De Homine (Des Cartes 1662; Descartes 1664a), whereas in subsequent editions the copperplate engravings were replaced with woodcuts and did not include the flap (Crummer 1932). At the top of the illustration is a probe in the cerebellomedullary cistern between the medulla and the midline cerebellum, possibly extending to the foramen of Magendie although not shown. Although the hemispheres have been separated, showing the midline sagittal view of each hemisphere, there is no sectioned corpus callosum or fornix shown on either side, which is an anatomic error.

In addition, Schuyl used superimposition as a technique to illustrate motion of the pineal gland, a technique also subsequently adopted by the illustrators of the French edition [Figure 10 and Figure 11].

The last illustration of the human nervous system in De Homine (Des Cartes 1662; Descartes 1664a) was focused strictly on the pineal gland (A), its corporeal–spiritual connection with the soul (B), and its extended ramifications expanding below the gland in a microscopic reticulum by which the pineal interacts with the nervous system (Des Cartes 1662, Figure 54 of Folio 118 and “Nota” following page 121) [Figure 12].

6. Descartes’ Bête-Machine Doctrine

Descartes’ views on the souls of animals were apparently not based on philosophical arguments concerning empirical evidence but were instead a theological and metaphysical doctrine apparently developed initially to be consistent with Scripture (Morris 2000).

In Descartes’ Discourse on the method (1637), Descartes suggested that it was possible to build automata (machines) that could not be distinguished from animals, whereas such machines could always be distinguished from humans by their lack of language and reasoning (Descartes 1637; 2001, pp. 45–47): “For it is a very remarkable thing that no men are so dull and stupid—not excepting even the insane—that they are not capable of arranging various words together, and making discourse from them through which they make their thoughts understood; and on the contrary, there is no other animal, no matter how perfect and as happily born, which can do the same thing” (Descartes 2001, p. 46). While some birds, such as magpies and parrots, have the vocal apparatus to pronounce words like humans, they nevertheless do not really understand or communicate these vocal utterances, whereas even humans who are born deaf and mute invent signs to allow them to be understood by close associates; for Descartes this was evidence that animals have “no reason at all” (Descartes 2001, p. 46). Furthermore, even though animals could be more industrious in some things than humans, for Descartes this was a simple, automatic, and mechanical process: “it shows that they have no minds at all, and that it is nature which acts in them, according to the disposition of their organs, as we see that a clock, which is composed of nothing but wheels and springs, can count the hours and measure time more accurately than we can with all the care at our command” (Descartes 2001, p. 46).

Descartes then addressed the rational soul and concluded that it could not come from the power of matter but must be expressly created and joined to the body in a very intimate manner:

[I]t would not suffice that [the soul] be placed in the human body, as is a pilot in his ship, unless perhaps to move its members. Instead, it is necessary that the soul be joined and united more closely to the body, [to have] feelings and appetites similar to ours, and thus compose a true man”

(Descartes 2001, p. 47)

This was the background upon which Descartes claimed to have established a religious doctrine that only humans have a rational soul, which is independent of the body and immortal.

After error of those who deny God … there is no other [error] which draws feeble minds away from the straight path of virtue so often than that of imagining that the soul of beasts is of the same nature as ours, and that as a result, we have nothing to fear nor to hope for after this life, any more than do flies and ants; whereas, when we know how much they differ, we have a much better understanding of the reasons which prove that our soul is of a nature entirely independent of the body, and as a result, that the soul is not subject to death along with the body. Then … we are naturally led to judge from this that it is immortal

(Descartes 2001, p. 48)

In a letter on 3 October 1637 to the Dutch physician and scholar Vopiscus Fortunatus Plempius (Plemp; 1601–1671), professor of medicine in Amsterdam, Descartes responded to some criticisms of the Liégeois14 theologian and natural philosopher Libertus Fromondus (Libert Froidimont; 1587–1653):

[Fromondus] asks what is the point of attributing substantial souls to animals, and goes on to say that my views will perhaps open the way for atheists to deny the presence of a rational soul even in the human body. I am the last person to deserve this criticism, since, like the Bible, I believe, and I thought I had clearly explained, that the souls of animals are nothing but their blood, the blood of which is turned into [animal] spirits by the warmth of the heart and travels through the arteries of the brain and from it to the nerves and muscles. This theory involves such an enormous difference between the souls of animals and our own that it provides a better argument than any yet thought of to refute the atheists and establish that human minds cannot be drawn out of the potentiality of matter. And on the other side, I do not see how those who credit animals with some sort of substantial soul distinct from blood, heat, and [animal] spirits, can answer such Scripture texts as Leviticus 17:14 (“The soul of all flesh is in its blood, and you shall not eat the blood of any flesh, because the soul of flesh is in its blood”) and Deuteronomy 12:23 (“Only take care not to eat their blood, for their blood is their soul, and you must not eat their soul with their flesh”). Such texts seem much clearer than others which are quoted against certain opinions which have been condemned solely because they appear to contradict the Bible. Moreover, since these people posit so little difference between the operations of a man and of an animal, I do not see how they can convince themselves [that] there is such a great difference between the natures of the rational and sensitive souls

(Descartes 1970, pp. 36–37 with punctuation edited; see also Descartes 1897, p. 412; 1939, p. 5)

In Descartes’ Meditations on first philosophy (1641), specifically in his reply to the fourth set of objections presented by French theologian, philosopher, and mathematician Antoine Arnauld (1612–1694), Descartes recapitulated what he wrote in his Discourse on the method (1637):

[I]f we wish by reasoning to determine whether any of the motions of brutes are similar to those which we accomplish with the aid of the mind, or whether they resemble those that depend alone upon the influxus of the animal spirits and the disposition of the organs, we must pay heed to the differences that prevail between the two classes… [A]ll the actions of brutes resemble only those of ours that occur without the aid of the mind. Whence we are driven to conclude that we can recognize no principle of motion in them beyond the disposition of their organs and the continual discharge of animal spirits that are produced by the beat of the heart as it rarefies the blood. At the same time we shall perceive that we have had no cause for ascribing anything more to them, beyond that, not distinguishing these two principles of motion, when previously we have noted that the principle depending solely on the animal spirits and organs exists in ourselves and in the brutes alike, we have inadvisedly believed that the other principle, that consisting wholly of mind and thought, also existed in them. And it is true that a persuasion held from our earliest years, though afterwards shown by argument to be false, is not easily and only by long and frequent attention to these arguments expelled from our belief

(Des Cartes 1641; Descartes 1955, p. 104)

Descartes reiterated his biblical argument in a letter to a Dutch official, Buitendijck, at the University of Dordrecht in 1643 regarding the soul of brute animals:

I would not want to say that movement was the soul of beasts, but rather, according to holy Scripture and Deuteronomy 12:23, that blood is their soul. For blood is a fluid body, which is very lively, and its more rarefied part[icle]s are called spirits. It is these, flowing from the arteries through the brain into the nerves and muscles, which move the whole machine of the body

(Descartes 1659a, pp. 53–54; see also Descartes 1901, p. 62; 1951, p. 82; 1970, p. 146)

As late as 1649, Descartes received a laudatory letter from Cambridge religious philosopher Henry More (1614–1687) to which he responded on 5 February 1649. Descartes noted their shared belief that there is a soul that moves the human body and thinks, and he acknowledged that animals are constructed similarly and have overlapping behavior with humans; hence, animals might reasonably be thought to have souls of their own. However, with remarkable hubris or arrogance, Descartes claimed to be able to explain apparent thinking in animals on a mechanical basis, allowing him to side-step the problem of assigning incorporeal souls to animals.

[T]here is no prejudice to which we are all more accustomed from our earliest years than the belief that dumb animals think. Our only reason for this belief is the fact that we see that many of the organs of animals are not very different from ours in shape and movement. Since we believe that there is a single principle within us which causes these motions—namely the soul, which both moves the body and thinks—we do not doubt that some such soul is to be found in animals also. I came to realize, however, that there are two different principles causing our motions; one is purely mechanical and corporeal and depends solely on the force of the spirits and the construction of our organs, and can be called the corporeal soul; the other is the incorporeal mind, the soul which I have defined as a thinking substance. Thereupon I investigated more carefully whether the motions of animals originated from both these principles or from one only. I soon saw clearly that they could all originate from the corporeal and mechanical principle, and I thenceforward regarded it as certain and established that we cannot at all prove the presence of a thinking soul in animals. I am not disturbed by the astuteness and cunning of dogs and foxes, or all the things which animals do for the sake of food, sex, and fear; I claim that I can clearly explain the origin of all of them from the constitution of their organs

(Descartes 1970, p. 243)

Descartes was driven by his preconceived ideas concerning the immaterial and immortal souls of humans, which he felt must have been specially granted to humans and not animals. Using fallacious reasoning and debate tricks, he argued that animals could not think and hence could not have immortal souls. Descartes acknowledged that he was unable to prove that animals have thoughts, or that they don’t, but by a reductio ad absurdum argument (i.e., argument from absurdity) he claimed that animals cannot have thoughts: if one accepted that some animals have thoughts (e.g., chimpanzees), that would imply (according to Descartes) that all animals have thoughts and hence immortal souls, even such lowly animals as worms and flies, which Descartes felt would be absurd. Descartes found it easier to imagine that worms and flies “move mechanically” and hence without thought or immortal souls, so this implied to him that all animals move mechically without immortal rational souls.

But though I regard it as established that we cannot prove there is any thought in animals, I do not think it is thereby proved that there is not, since the human mind does not reach into their hearts. But when I investigate what is most probable in this matter, I see no argument for animals having thoughts except the fact that since they have eyes, ears, tongues, and other sense-organs like ours, it seems likely that they have sensation like us; and since thought is included in our mode of sensation, similar thought seems to be attributable to them. … But there are other arguments, stronger and more numerous, but not so obvious to everyone, which strongly urge the opposite. One is that it is more probable that worms and flies and caterpillars move mechanically than that they all have immortal souls

(Descartes 1970, p. 244)

Descartes further argued that the bodies of humans and those of animals can move in all the ways that have been observed, even in the absence of thought, and hence without the intercession of the soul: “This is very clear in convulsive movements when the machine of the body moves despite the soul, and sometimes more violently and in a more varied manner than when it is moved by the will” (Descartes 1970, p. 244). Therefore, observing any possible motion of a human or animal body did not establish the presence of an immortal soul. Further, since men can make “various automata” that move without thought (i.e., moving mechanical devices made to imitate some aspect of the observed behavior of a living person or animal), “it seems reasonable … that nature should produce its own automata [i.e., without souls], much more splendid than artificial ones [made by men]—These natural automata are the animals” (Descartes 1970, p. 244 with punctuation edited). Descartes concluded that “it is much more wonderful that a mind should be found in every human body than that one should be lacking in every animal” (Descartes 1970, p. 244), but nothing he presented proved either of these assertions.

Descartes’ final and most important test of thought—and therefore of an immaterial soul—as a distinguishing factor between humans and non-human animals, is the presence of “real speech” exclusively in humans. Of course, at that time, he was unable to imagine that “real speech” could be imitated by mechanical devices. Descartes also never considered the full implications of “real speech” as a marker of the presence of a soul: Did an immaterial soul not join the body until a child has acquired “real speech”, and did an immaterial soul leave the body before death if a person became unable to communicate due to profound neurologic illness (e.g., coma, end-stage neurodegenerative conditions, locked-in syndrome, etc.)?

[I]n my opinion the main reason which suggests that the beasts lack thought is the following. … [A]lthough all animals easily communicate to us, by voice or bodily movement, their natural impulses of anger, fear, hunger and so on, it has never been observed that any brute animal reached the stage of using real speech, … of indicating by word or sign something pertaining to pure thought and not to natural impulse. Such speech is the only certain sign of thought hidden in the body. All men use it, however stupid and insane they may be, and though they may lack tongue and organs of voice; but no animals do. Consequently, it can be taken as a real specific difference between men and dumb animals

(Descartes 1970, pp. 244–45)

7. Schuyl’s Textual/Figural Inconsistencies Concerning the Bête-Machine Doctrine

As Schuyl expressly stated in an appended “Nota”, two of his gross brain illustrations in De Homine (Des Cartes 1662; Descartes 1664a) concern goat brains, both of which are here presented with photographs of comparable ungulate (sheep) brains to show the relevant anatomy [Figure 13 and Figure 14]. One of Schuyl’s illustrations is a dorsal view showing the cerebrum and cerebellum, with the frontal pole at the bottom of the illustration [Figure 13 left], and the other is a postero-dorsal view (with the frontal pole facing away from the viewer, though not shown) with the brainstem artificially flexed to reveal the seed-shaped pineal gland, two enormously enlarged ball-like superior colliculi, two more flattened inferior colliculi below them, and then the upper surface of the cerebellum at the bottom of the illustration [Figure 14 left]. Although the superior colliculi are comparatively larger in ungulates than in humans, their size has been greatly exaggerated in these figures. To facilitate viewing the same structures in a dissected sheep brain, the corpus callosum was sectioned, and the brainstem was flexed a milder amount over the handle of a scalpel [Figure 14, right].

What is notable, and puzzling, about these two figures by Schuyl is that both include, in Schuyl’s characteristic manner, a drape-like representation of the corporeal–spiritual connection of the pineal gland with the soul. In the dorsal view [Figure 13 left] it extends through a hole-like gap between the cerebral hemispheres and then tilts toward the right (to make it easier to visualize in this dorsal view) and in the view through an artificially flexed brainstem [Figure 14 left], it extends superiorly from the pineal gland through an imaginary hole in the splenium of the corpus callosum. Since there is no actual anatomic structure with these characteristics, in either ungulates or humans, the purpose of its illustration in Schuyl’s engravings is not representational. It is also clearly not simply decorative. Instead, its purpose is nonrepresentational (in an artistic sense) or metaphysical (in a philosophical sense)—to express something of relevance and importance that is not visible. Given the persistence of this motif in Schuyl’s engravings, its uniform graphical linkage to the pineal gland, the manner of its visual representation (like a fabric billowing in the breeze), and Descartes’ expressed ideas of how the pineal gland serves (in man) as the linkage between the corporeal body with the spiritual (and hence invisible) soul, this seems to be the inescapable significance or particular purpose of this graphic device.

The problem is that these figures clearly contradict Descartes’ own views on animals—expressed in his Discours de la methode (1637), Meditations on first philosophy (1641), and correspondence, with an expanded discussion promised for, but not provided in, the posthumous Treatise on Man—that brute animals lack not only reason, but any form of self-aware consciousness, since they have neither mind nor a rational soul; this is the so-called bête-machine (beast-machine) doctrine (Descartes 1637; Des Cartes 1641; Rosenfield 1968; Massey and Boyle 1999; Morris 2000; Newman 2001; Park 2012, 2017; Cole and Markley 2014; Vermij 2024).

Schuyl himself, in an extended 33-page preface (Ad Lectorem, i.e., To the Reader), offered the first detailed published defense of the animal machine concept, a defense that brought the issue to international attention, especially after Clerselier included an abridged French translation (by Clerselier’s son) of Schuyl’s essay in L’Homme two years after Schuyl’s version appeared in De Homine (Des Cartes 1662; Descartes 1664a, 1664b, 2003; Vermij 2024). Schuyl’s preface has the character of a separate treatise with relatively little reference to Descartes’ text, indicating that it was more than simply an introduction to Descartes’ monograph (Vermij 2024). Indeed, Schuyl was apparently responding to an ongoing contemporary religious/political controversy in which aspects of Cartesian philosophy, such as the beast machine concept, served as proxies for wider political and ecclesiastical issues (Vermij 2024).

Schuyl focused on what is now referred to as the soul–body problem, providing scriptural, theological, and quasi-biological arguments against the ancient Greek idea that animals have souls, at least of the conscious, rational sort present in man. For both Descartes and Schuyl, the observed activities of animals are attributable not to an immaterial rational soul but are instead due entirely to the organization of their material bodies. In contrast, vegetative functions of animal and human bodies occur without the intervention of a rational soul by the corpuscular dynamics of Cartesian physiology. Thus, as the late American zoologist, historian of science, and Cartesian scholar Thomas Steel Hall (d. 1990)15 succinctly expressed, “Descartes thus saved for the soul that divine—and consciously cognitive role—that ecclesiastical dogma demanded” (Descartes 2003, p. xlii).

Because Schuyl so closely followed Descartes’ arguments concerning the question of whether animals have souls, he was evidently quite familiar with Descartes’ Discourse on the method (1637), Meditations on first philosophy (1641), and the initial volumes of Descartes’ correspondence published by Clerselier.

Like Descartes had in correspondence, Schuyl framed a major portion of his argument as a defense of Christian piety, using the same biblical passages (Leviticus 17:14 and Deuteronomy 12:23) and even adding others (e.g., Genesis 9:4). As Schuyl wrote in his preface,

[Descartes] wanted to destroy that most terrible opinion, which, by a kind of wicked metamorphosis and metempsychosis,16 strives to change men into beasts and beasts into men, desecrating by excessive affinity with brutes the incorporeal and incorruptible mind by which man is called by special prerogative the image of God.

(Schuyl’s Ad Lectorem in Des Cartes 1662, although the preface is not paginated, this quotation is from the 4th page of the preface; see also Vermij 2024, p. 201).

Schuyl continued his sermon by alleging that the Devil had always done his work through philosophers who argued that “our race differs nothing from brutes”. Schuyl included many earlier philosophers as agents of the Devil for failing to sufficiently distinguish the souls of humans and animals, specifically citing Zoroaster (also called Zarathushtra Spitama, or Zarathustra; c. 624-599 BCE—c. 547–522 BCE), Pythagoras (c. 570 BCE—c. 495 BC), Anaxagoras (c. 500 BCE—c. 428 BC), Plato (c. 428—423 BCE—348/347 BCE), Aristotle (384 BCE—322 BCE), Pliny (the elder; 23—79 CE), Plutarch (c. 46 CE—120s CE), and Porphyry (234 CE—305 CE).

Schuyl added other quasi-biological philosophical arguments that implied absurdities if animal souls exist: (1) the then-current belief in spontaneous generation of some animals would imply the spontaneous generation of their putative souls; and (2) the fact that some animals (e.g., centipedes) can be divided into parts yet still show signs of life would imply that an animal soul was divisible, and hence material (based on then-current beliefs about human souls).

Schuyl’s defense of Descartes’ bête-machine doctrine, but his seemingly contradictory illustrations of a corporeal–spiritual connection of the pineal gland with the soul in goats, begs several questions. What was the purpose of his extended preface? And did Schuyl personally believe what he wrote (or illustrated)?

While only a partial tentative explanation can be given, Schuyl was apparently not concerned with any major philosophical issues about either the mind or the soul (of man and animals) but was instead writing a defense of Descartes’ concepts to preclude or temper religious and political objections that had been circulating in the Dutch context (Vermij 2024). As Vermij cogently argued, “Schuyl’s preface is not so much a contribution to a scholarly debate, but to a political controversy and its context is to be found not in learned treatises, but in vernacular pamphlets [that concerned] the position of the Church in society, the definition of religious orthodoxy, and the correct interpretation of the Bible” (Vermij 2024, p. 194). The largely textual debate focused on points where Descartes’ views seemed to directly contradict either the Bible or common sense (e.g., the apparent cognitive and emotional abilities of some animals did not seem automatic or mechanical for many observers). Schuyl (as Descartes had earlier) strove to show that the Cartesian framework was not only consistent with the Bible but that it could account for things that seemed to defy common sense.

8. Discussion

Descartes had selected the pineal gland as the locus for the corporeal–spiritual connection of the body with the immaterial and immortal soul because it was, he thought, the only locus that met his preconceived constraints: an unpaired structure within the brain that could direct animal spirits for both perception and motor activity. Nevertheless, for the pineal mechanisms to work as Descartes stated, he had to mislocate the pineal gland within the third ventricle. The pituitary gland, although also unpaired, was unsuitable, because, as a carryover from ancient Greek medicine—specifically from Hippocrates of Cos (460 BCE—370/375 BCE) and later Galen of Pergamon (129 CE—210/216 CE)—the pituitary was considered an organ for the drainage of waste from the brain, to eventually be discharged through the nasal passageways as phlegm.17

Moreover, for Descartes to deny that animals have a rational soul—as Descartes believed must be the case from his reading of Scripture—the distinction between man and animals had to be discrete and absolute: (1) man can communicate through language whereas animals cannot; and (2) man has reason, while animals do not. There could be no middle ground where some animals could communicate in a more limited way through language, or where some animals could show evidence of reasoning or problem-solving, even if not as sophisticated as “even the stupidest humans”. For Descartes to admit either of these, for some animals to even a modest degree, would undermine his ability to reserve for man alone an immortal soul. This forced Descartes to argue for a black-and-white separation between man and all animals in the use of language and reasoning capability. He even stooped to ad hominem attacks on those who believed otherwise, labeling them variously as atheists, agents of the Devil, or weak-minded. Thus, he was arguing from a religious or metaphysical perspective, and not an empirical framework. His primary purpose was to explain the world in a manner consistent with Christian doctrine, regardless of how much he tried to dress it up as a logical derivation from first principles evident to any thoughtful person.

The preeminent role for the pineal gland in Cartesian sensorimotor processes was well illustrated by Descartes’ acolytes in both the posthumous Latin and French editions of Descartes’ Treatise on Man (Des Cartes 1662; Descartes 1664a, 1664b). However, only Schuyl, in the little-studied Latin edition, attempted to illustrate Descartes’ imagined corporeal–spiritual connection of the pineal gland with the immaterial and immortal soul; Schuyls’ repeated illustration of this corporeal–spiritual connection has apparently been unrecognized previously, presumably because the illustrations of the Latin edition have received little scholarly attention (Wilkin 2003; Chan 2016; Nadler 2016, 2024).

Schuyls’ extensive preface (Ad Lectorem) to De Homine (Des Cartes 1662; Descartes 1664a) was the first detailed published defense of Descartes’ bête-machine (beast-machine) doctrine (Vermij 2024), as expressed in Descartes’ Discours de la methode (1637), Meditations on first philosophy (1641), and correspondence. Schuyl’s written stance in his preface—arguing largely on religious grounds that animals do not have a conscious, rational soul—was consistent with Descartes’ earlier arguments but contrasted sharply with Schuyl’s illustrations of a corporeal–spiritual connection of the pineal gland with an immaterial soul in goats. The inconsistency between Schuyl’s textual preface and his illustrations suggests that Schuyl either (1) had some personal doubts about whether animals have souls, despite his strongly worded defense of the bête-machine doctrine; (2) had different audiences in mind for his textual and figural messages, possibly feeling that a written denouncement of the possibility that animals have souls was necessary to avoid or minimize controversy; (3) changed his mind after he had devoted so much time to creating the images; or (4) some combination. Various authors of this period (and well into the 18th century) incorporated verbiage in their texts solely to allow publication or to stem arguments and opposition that could be predicted based on existing religious beliefs. Although no definitive answer can be expected, it may be that Schuyl’s preface was meant to stave off arguments and opposition and that Schuyl’s illustrations more closely reflect his actual beliefs. Regardless of the actual (unknowable) explanation, the textual/figural discordance reflects two diametrically opposed stances that Schuyl had considered on the question of whether animals have souls.

Schuyl’s creative use of illustrations was not limited to the depiction of the corporeal–spiritual connection of the pineal gland with the immaterial and immortal soul but extended to the illustration of dynamic changes using layered static images. Indeed, Schuyl illustrated Descartes’ idea of the mobility of the pineal gland using a seldom recognized, seldom discussed, and rarely shown flap anatomy (Crummer 1932). Schuyl also used a flap anatomy in a more widely recognized copperplate illustration of the heart, which had two flaps representing windows cut into the right and left ventricles (not shown here) (Crummer 1932). Physician and medical historian LeRoy Crummer (1872–1934) considered Schuyl’s flap anatomies to be unique because “both front and back of the plates show details of marvelously executed copperplate engravings; in other words, when the flaps are lifted, the details shown on the inside are just as accurate as the details showing with the flap in position” (Crummer 1932, p. 138). There were no flap anatomies in the French edition of Descartes’ Treatise on Man (Descartes 1664b).

In late 1664—the same year as the publication of the French edition of Descartes’ Treatise on Man—Danish anatomist, natural philosopher, and later theologian Niels Stensen began a series of brain dissections in Paris [Figure 15]. In April 1665, he gave a lecture on brain anatomy in Paris to an audience of avowed Cartesians, which addressed and refuted Cartesian dogma regarding the pineal gland. Stensen showed that Descartes’ localization of the pineal gland within the ventricle was fallacious, as were his imaginations concerning the mobility of the pineal gland (e.g., as illustrated in the little-known flap anatomy by Schuyl). As Steno demonstrated, in contrast to Descartes’ stated views, the pineal gland is a midline structure located outside of the substance of the brain, attached by a stalk to the external surface of the posterior wall of the third ventricle near the superior colliculi of the midbrain tectum. That is, the pineal gland is situated outside of the brain parenchyma and outside of the ventricular system, facts which undermined Descartes’ imagined physiology of the brain, and his justification for the pineal gland as the corporeal–spiritual connection of the body with an immaterial and immortal soul. Stensen’s lecture was surprisingly well accepted given the Cartesian orientation of the audience. His lecture was subsequently published in Paris in 1669 as Discovrs de Monsievr Stenon, svr l’anatomie dv cerveav [Steno’s lecture on the anatomy of the brain] (Stenon 1669; Steno 1965, 2013; Andrault 2016).

Figures in manuscripts and printed books can be considered as a component of the “paratext,” a concept initially developed and expounded by French literary theorist Gérard Genette (1930–2018) (Genette 1987, 1997). Paratext is the other material that supports the main text, which is supplied by the authors, editors, illustrators, printers, and publishers, including the illustrations. As Genette explained, the paratext “is at the service of a better reception for the text and a more pertinent reading of it” (Genette 1997, p. 2). Indeed, illustrations can change the reception of a text or its interpretation by the public and sometimes can have a significance well beyond that of the main text, as did the illustrations from Descartes’ oeuvre concerning the mechanical model he proposed for the functions of the brain (Descartes 1637, 1664a, 1664b; Des Cartes 1662). Paratext is composed of peritext and epitext, where peritext is the collection of non-text components of a published work (e.g., illustrations) and epitext is material that falls “outside of the text” but that nevertheless concerns it—for example, preliminary drafts, correspondence concerning the text or figures, critical reviews of the work, and the author’s responses to such critiques (Genette 1987, 1997; Tweed and Scott 2018). As demonstrated above, Cartesian physiology is more clearly understood by combining Descartes’ texts (e.g., Treatise on Man) with their extensive paratext, comprised of corresponding peritext (e.g., illustrations) and extant epitext (e.g., correspondence and critiques).18