The Description of the Human Body (AT XI 223–86, extracts CSM I 313–24) was written in French in the winter of 1648 (see AT V 112, CSMK 329). The manuscript was published posthumously by Clerselier in 1664 with the Treatise on Man. It contains five unequal parts: a preface, a second part dealing with the movement of the heart and of the blood, the third with nutrition and aging, and the last two parts with the formation of the animal. The second part gives detailed anatomical and physiological explanations linked with the Treatise on Man and even more with the fifth part of the Discourse on Method, especially the relation to Harvey's demonstrations of the movements of the heart and of the blood in his Latin treatise of 1628. Some explanations are summarized in the first part of the Passions of the Soul. The Description was published with the subtitle “On the Formation of the Fetus,” with Clerselier pointing out the importance of embryology or, in seventeenth-century terms, of the generation of animals. Descartes had given up explaining this question in the Treatise on Man (AT I 254, CSMK 39) but returned to it later (see his Latin fragments, Thoughts about the Generation of Animals, AT XI 505–38).

At the beginning of the Description, Descartes evokes the usefulness of the ancient motto “Know thyself,” in order to cure illness and to prevent it, and also to avoid “attributing to the soul the functions which depend solely on the body and on the disposition of its organs” (AT XI 223, CSM I 314). After defining the human mind in terms of thought (AT XI 224, CSM I 314), an echo of the Meditations and of the Principles I.9, Descartes gives an account of “the entire bodily machine” that establishes a dissociation between reasoning about anatomy and physiology, on the one hand, and teleological or theological considerations, on the other, as shown by the study of the motion of the heart and of the blood (AT XI 226, CSM I 315).