Substance and what it means to be › Capacity Is Not the Same as Its Exercise

Book IX.3 addresses a position attributed to the Megaric school: a thing can act only when it is acting. Aristotle's builder supplies a counterexample. A builder does not lose the art of building whenever the work stops and acquire it again when work resumes. Possessing a capacity and exercising it must therefore be distinguished.

In IX.6 Aristotle explains actuality by analogies, including building compared with being able to build, and being awake compared with being asleep. He does not restrict the distinction to a single model of physical change. Potentiality concerns what something can be or do, while actuality concerns its being or activity as realized. These terms require attention to the relevant capacity and circumstances. They do not turn every imaginable future into a real possibility for the thing under discussion.

For a contemporary illustration, imagine a pianist waiting silently before a performance. The silence alone does not establish that she lacks the skill to play. Conversely, wanting to perform a difficult piece is not sufficient to possess that skill. The distinction helps separate an available capacity from an aspiration and from its present exercise. This example draws a conceptual lesson from Aristotle; it makes no claim about experimental psychology.

In IX.8 Aristotle argues that actuality is prior to potentiality in account and substance, while treating temporal priority with qualifications. We understand a capacity by reference to the activity for which it is a capacity. A particular developing thing may precede its own mature activity in time, yet Aristotle also appeals to something already actual in explaining its development. The claim is therefore more precise than saying that every actual state always occurs earlier than every potential one.