Abstract

My thesis examines Aquinas's philosophical psychology in the context of his controversy with Siger of Brabant. Chapters 1 and 2 are concerned with the central thesis of Aquinas's argument against the Averroist doctrine of the intellect, which he often expresses by the claim, intellectus est forma corporis. This claim is fundamental to Aquinas's philosophical psychology insofar as it relates not only to his solution to the tension between the Aristotelian definition of the soul as the form of the body and the intellect's immateriality, but also to his alternative to the Averroist interpretation that the intellect cannot be the form of the body. I argue that this claim cannot be an adequate alternative to the Averroist interpretation of Aristotle since it is not compatible with Aquinas's insistence that the intellect cannot use any bodily organ, nor exist in any part of the body. Furthermore, the thesis of organlessness commits Aquinas to an intellect-body dualism since he postulates the existence of the intellect independently of the body in the human being. ;Chapters 3 and 4 deal with the question of whether Aquinas has an adequate alternative to Siger's critique that the intellect cannot be a power of the human soul which is the substantial form of the body. Some contemporary Thomists have argued that Aquinas's position is not vulnerable to the Sigerian critique at all by appealing to his claim that the human soul is a subsistent form. However, I maintain that the human soul's subsistence cannot be upheld insofar as Aquinas insists that the human soul is essentially the form of the body, and its nature is incomplete without the body. For nothing in the nature of the human soul as the form of the body provides a ground for showing that it can perform its proper operation without the body and thus can be subsistent. Consequently, Aquinas does not offer a satisfactory explanation of the possibility that the human soul can possess the intellect as its power. This leads us to conclude that the difficulties raised by Siger cannot be resolved by Aquinas's position