 In the later Middle Ages the study of the soul was part of natural philosophy. Marsilius treated the human soul in his commentary on Aristotle’s De anima, in which he followed the Parisian tradition of Buridan and Oresme concerning the particular questions addressed. Following Buridan, he argued that there is no natural proof of the immortality of the human soul. For the human natural mind, unaided by revelation, the theory of Alexander of Aphrodisias that the human soul is corruptible is the most probable. That Alexander of Aphrodisias is mistaken and that the soul continues to exist after the death of the body is known through revelation alone. Faith has more authority than human reason and must be accepted in all cases where the two conflict since the things we believe on faith come from God, who cannot  err.[22]