Fermentation as ‘Heat-Rarefaction’ and Animal Spirits in Descartes’ Medical Philosophy

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Crucial to Descartes’ physiology in general, fermentation is also at the root of muscular movement and embryo formation. I argue that in contrast to theories of fermentation popular with chymists, Descartes’ own version of how fermentation happens is best described as a ‘heat-rarefaction’ model, whose essential components are the presence of heat and its rarefying effect on matter, resulting in an expansion in volume. This is an eminently mechanist model, which resonates most closely with certain corpuscularist medieval reworkings of ancient Aristotelian and Galenic ideas. Descartes’ use of fermentation is traced across writings such as Treatise on Man, Description of the Human Body, Passions of the Soul, Primae Cogitationes, Excerpta anatomica, and Descartes’ correspondence including some sections of the 1638 Plempius letters omitted in previous translations. By explaining the misunderstanding arising out of different ways of conceptualizing fermentation, this hypothesis sheds new light on the tensions between Descartes and some of his contemporaries, especially Plempius. It also shows how ‘fermentation’ is negotiated between chymists and mechanists around the middle of the seventeenth century. While both schools of thought see fermentation as a model for processes happening in the natural world, each ascribes fermentation a different manner of action in matter.