Abstract
The concept of entropy is mildly consensual on two apparently paradoxical points: it is a fundamental concept of science, and nobody knows what it really is. Historically, it reveals two great ambitions: to account for the impossibility of perpetual motion in the macroscopic realm and to derive this account from the microscopic, dynamical laws. The result is a baffling diversity of definitions and interpretations. In this article, we review five of these interpretations: as disorder, unavailability of work, probable state, missing information, and spreading, while discussing their merits in their scientific, historical and philosophical dimensions. The discussions will assess the strengths and weaknesses of these interpretations while highlighting original insights regarding irreversibility, probability, emergence, objectivity and ‘levels of reality’. It concludes that, as a polysemous and fragmented concept, no slogan can capture the subtleties of entropy, but these insights provide a path to a better comprehension of entropy.