  In the mid-1640s, Descartes continued work on his physiological system, which he had pursued throughout the 1630s. He allowed his Treatise on Man to be copied (4:566–7) and  he began a new work (5:112), Description of the Human Body, in which he sought to explain the embryonic development of animal bodies. During this period he corresponded with Princess Elisabeth, at first on topics in metaphysics stemming from her reading of the Meditations and then on the passions and emotions. Eventually, he wrote the Passions of the Soul (1649), which gave the most extensive account of his behavioral physiology to be published in his lifetime and which contained a comprehensive and original theory of the passions and emotions. Portions of this work constitute what we have of Descartes' moral theory.