3. - Thomas Aquinas (d. 1274), Commentary on the Sentences, book I, d. 3, q. 4, a. 2:

Summary

In this text, Aquinas discusses the question of whether the powers of the soul are the same as the soul. The text is part of Aquinas’ analysis of Augustine’s doctrine of the image of the Trinity. Like his teacher Albert the Great, he argues that the soul and its powers are distinct, and, like Albert, he holds that the soul’s powers are to be viewed as necessary accidents “flowing” from the soul. But Aquinas goes beyond his teacher in an important respect. He devises a new argument in favor of the distinction theory—an argument that we may call the Category Argument. The argument goes like this. A power must be in the same category as its act. All acts of the soul, like thinking, seeing, etc. are accidents. Hence, the powers of the soul must be accidents too. Aquinas justifies the major premise that a power must be in the same category as its act by appeal to what we might call a Causal Proportionality Principle. According to this principle, a cause must be like its immediate effect, where the cause is a power, and the immediate effect is its operation. The Category Argument proved very influential for the subsequent debate over the relation between the soul and its powers.