Abstract
Descartes rejects appeals to final causes in natural philosophy. Nonetheless, he employs the apparently teleological language of 'functio' and 'usus' in his physiology. I examine this interpretive problem by turning our attention especially to the explanatory structures Descartes uses and the medical context of his physiology. I employ analyses of 'function' in philosophy of biology to clarify my interpretive claims. Descartes primarily employs 'usus' and 'functio' to provide "function-analytical" explanations of the complex behavior of living things-a familiar project present also in the work of physician Jean Fernel. Here usus are treated like "Cummins-functions." However, Descartes occasionally employs final causal explanations like those in the work of anatomists Hieronymus Fabricius ab Aquapendente and William Harvey. Here usus are treated like "Wright-functions." Examples include Descartes's explanations of the bicuspid mitral valve in the heart and of the psycho-corporeal physiology of sensation. This kind of explanation is problematic. Descartes's explicit strategy for grounding such explanations in divine non-deceptiveness has significant difficulties.