  Descartes was known among the learned in his day as a top mathematician, as the developer of a new and comprehensive physics or theory of nature (including living things), and as the proposer of a new metaphysics. In the years following his death, his natural philosophy was widely taught and discussed. In the eighteenth century aspects of his science remained influential, especially his physiology, as did his project of investigating the knower in assessing the possibility and extent of human knowledge; he was also remembered for his failed metaphysics and his use of skeptical arguments for doubting. In the nineteenth century he was revered for his mechanistic physiology and theory that animal bodies are machines (that is, are constituted by material mechanisms, governed by the laws of matter alone). The twentieth century variously celebrated his famous “cogito” starting point, reviled the sense data that some alleged to be the legacy of his skeptical starting point, and looked to him as a model of the culturally engaged philosopher. He has been seen, at various times, as a hero and as a villain; as a brilliant theorist who set new directions in thought, and as the harbinger of a cold, rationalistic, and calculative conception of human beings. Those new to the study of Descartes should engage his own works in some detail prior to developing a view of his legacy.