This thesis examines the Summa de anima of a thirteenth-century writer, Jean de la Rochelle (1190/1200-1245), a philosopher and theologian of the Franciscan Order. The importance of the text lies in the fact that it was written at the very early stages of the changes that took place as a consequence of the new Greek and Arabic sources becoming available at this time. Of particular interest in this regard is Jean's precise exposition of his sources, in particular his study of the powers of the soul according to the Pseudo-Augustine, John Damascene and Avicenna. It can be seen, however, that despite the connections made between A vicenna and Augustine, a tension emerges, one which was to become much more pronounced when the works of Aristotle, particularly his De anima, were made available to the medieval schoolmen. The question of the unity of body and soul was not 'the' question for a theologian of Jean's generation. The challenge facing our author was how to distinguish the intellective soul from the composite body in order to argue for the immortality of the soul. The new physiological understanding of man, coming from the Greek-Arab sources, simply had to be confronted and taken into consideration. This was, at least, a turning towards the complex philosophical question of how sensing did not only involve having physical sensations but that our nlental states were also affected by the experience. While Jean brings out the difference between sensations according to actions, objects, and organs of the body, the connection between sensations and physiological states was not, at this speceific time, the problem for Jean and his contemporaries. Jean's Summa, however, does provide the ground for the mind-body problem in later thinkers, in particular, Thomas Aquinas. The study includes an Appendix of Latin text with the English translation of selected passages from Jean's Summa.