At various points in his work on physiology and medicine, Descartes refers to a “principle of life.” The exact term changes—sometimes, it is the “principle of movement and life”, sometimes the “principle underlying all [the] functions” of the body —but the message seems consistent: the phenomena of living bodies are the product of a single, underlying principle. That principle is generally taken to be cardiac heat.1 The literature has, quite reasonably, taken this message at face value. Thus, Shapiro: “Descartes insists (...) again and again that the human body is properly to be described as a machine whose workings are […] driven by the heat in the heart that... (shrink)