Having and exercising a capacity
A builder does not lose the ability to build whenever construction stops; a sighted person with closed eyes differs from a blind person. These examples oppose identifying capacity with present activity.
A capacity is specified for an activity and under conditions. It does not establish every imaginable outcome, and wishing for a result does not supply the relevant objects or circumstances.
References: [1]
Rational capacities and alternatives
Book IX distinguishes rational skills from non-rational powers. A body of knowledge can concern contrary results, as medical knowledge addresses health and disease, without producing both simultaneously.
Desire, choice and the condition of the object help determine the activity performed. This is Aristotle’s philosophical account of capacities rather than a universal law established by modern science.
References: [1]
Materials, activity and analogy
Examples include a statue that can be made from wood and a person capable of study compared with one studying. The relationships are analogical; a potential object is not simply a miniature finished object concealed inside material.
The text also distinguishes changes directed toward a completion from activities such as seeing. Potentiality and actuality therefore concern modes of being and activity, not merely earlier and later moments.
References: [1]