Descartes repeatedly refers to a “principle of life” and appears to make grand claims for its role in his natural philosophy. These claims have been taken at face value in the literature. This paper argues that there is no single principle underlying the operation of the Cartesian body. I show that Descartes’s account of physiology explains the operation of the living body through multiple interdependent systems, with no one system more fundamental than any other. As such, Cartesian physiology is incompatible with a hierarchical conception of a body whose operations are driven by a single underlying principle.