During the Scientific Revolution one important gauge of the quality of reformed natural philosophical knowledge was its ability to produce a more effective medical practice. Indeed, it was sometimes thought that philosophers who pretended to possess new and more potent philosophical knowledge might display that possession in personal health and longevity. René Descartes repeatedly wrote that a better medical practice was a major aim of his philosophical enterprise. He said that he had made important strides towards achieving that aim and, on that basis, he offered practical medical advice to others and advertised the expectation that, taking his own advice, he would live a very long time. This paper describes what Cartesian medicine looked like in practice and what that practice owed to the power of modernist Reason.