The distinction between dispositions and powers was drawn partly in terms of their subjects : it is a necessary condition on a power, but not on a disposition, that it be a property of an agent. The distinction between powers generally and abilities in particular may be drawn in terms of their objects . A power is an ability just in case it relates an agent to an action .
Some examples may make this distinction clear. Some powers, though properties of agents naturally expressed by ‘can’, do not intuitively involve any relation to action. The case of understanding, just mentioned, is a good example of this. Understanding a sentence, while it is not wholly passive or arational, is not typically an action. In contrast, speaking a sentence is. Thus the power to understand French will be a power, but not an ability, on the present taxonomy. In contrast the power to speak French will be an ability, since it involves a relation to actions.
This way of drawing the distinction inherits the problems involved in drawing the distinction between actions and non-actions. First, there is the problem that the domain of action is itself a contentious matter. Second, there is the problem that, even if we have settled on an account of action, it is plausible that the domain of action will be vague, so that there are some events that are not definitely actions, but that are not definitely not actions either. If this is right, then the present account of ability, which is cashed out in terms of action, will be correspondingly contentious and vague. The cases borderline between action and non-action may generate problems for the theory of ability. But such problems will not be central here. For giving such a theory will be difficult enough even when we focus on paradigm cases of action, and so on paradigm cases of ability.
Note there is a similarity between the present distinction between powers and abilities and the traditional distinction between intellectual and active powers, with the latter being powers that essentially involve the will, and the former those that do not (Reid 1785 & 1788). But it is not clear that these distinctions overlap exactly. For example, the power to will itself will clearly be an active power. It is less clear whether it will count as an ability, for the answer to that question will turn on the contentious question of whether willing is itself an action.