Q77 is perhaps the most difficult question in the Treatise, at least at the entry level. The issues it raises about “the capacities of the soul in general” look abstruse and uninviting, concerning matters such as the nature of a capacity (§5.1). and the relationship between such capacities and the soul's essence (§5.2). But although Q77 does not raise the obviously big issues found in other parts of the Treatise, it would be a mistake to dismiss this question as a mere metaphysical balancing of the books. The ideas developed here are fundamental to Aquinas's thinking about human nature, not just in 1a but in the later treatments of virtue that dominate the Summa's second part. The distinction between the soul's capacities and its essence would later prove to be one of the most controversial aspects of the theory (§5.3). Without this distinction, the Treatise would face a crippling circularity (§5.4). Moreover, Aquinas's focus on the soul's capacities is a consequence of an important methodological principle: that the soul's essence is not directly knowable (§5.5).

What is a capacity?

In the De anima, Aristotle offers the following remark in the course of making a transition from his general treatment of the soul to his treatment of the individual powers of the soul: