  Descartes also wanted to provide an account of the formation of plants and animals by mechanical causes, but he did not succeed during his lifetime in framing an account that he was willing to publish (so that only portions of his physiology were revealed in the Discourse, Dioptrics, Meditations, Principles, and  Passions). In writings that were published only posthumously (but were read by friends and followers during his lifetime, e.g., 5:112), he developed an extensive physiological description of animal bodies, in which he explained the functions of life in a purely mechanical manner, without appeal to a soul or vital principle.