 range of the possibilities inherent in things. Especially notable, for present purposes, are those that are intimately connected to agency. These include the susceptibility of things to be acted on in certain ways — such as the edibility of an apple, or the walkability of a trail — that the psychologist J.J. Gibson called affordances (Gibson, 1979). These include also the powers of action that we ascribe to things, of the kind observed by Thomas Reid: ‘Thus we say, the wind blows, the sea rages, the sun rises and sets, bodies gravitate and move’ (Reid 1788/2010, 16; Reid himself regarded these locutions as "misapplications" of active verbs, based on erroneous views of the grounds of powers). Finally, and crucially, these include the powers of agents themselves.
In light of this ontological diversity, it will be useful to have a term that encompasses all the possibilities inherent in things and in agents. Let us reserve the word ‘power’ for that general class. Dispositions, as defined above, are a proper subset of powers more generally. Affordances, as sketched above, are another one. And abilities, in a sense still to be defined, are yet another.
It may yet be that dispositions are especially privileged among the powers. For instance, they might be more fundamental than the other powers, in the sense that other powers may be reduced to them. It has been proposed, for instance, that affordances may be reduced to dispositions (Scarantino 2003). And we will consider in some detail, in Section 5.1, the proposal that abilities themselves may be reduced to dispositions. But our initial hypothesis is that dispositions are simply one member of the broader family of powers, albeit one that has received a great measure of attention in the philosophical literature.
Abilities, then, are a kind of power. What kind of power, precisely, is an ability? As the term will be understood here, there are two additional conditions that a power must meet in order for it to be an ability. First, abilities are distinguished by their subjects. Abilities are properties of agents, rather than of things that are not agents. Objects have dispositions and affordances — as a glass is disposed to break when struck, or affords the drinking of liquid — but they do not have abilities. Being a power of an agent is not, however, a sufficient condition for being an ability. This is because agents have powers that are not abilities. Therefore, second, abilities need to be distinguished by their objects: abilities relate agents to actions.
Some examples may make the need for this second condition clear. Some powers, though properties of agents, do not intuitively involve any relation to action. Consider the power of understanding language. 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 not be an ability, on the present taxonomy. In contrast the power to speak French will be an ability, since it involves a relation to action. (See van Inwagen 1983, 8–13.)
So, as the term will be understood here, an ability is a power that relates an agent to an action. This way of demarcating abilities, while serviceable for our purposes, is not unproblematic. For it 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. Arguably both of these points about action apply, also, to the property of being an agent. If this is right, then the present account of ability, which is cashed out in terms of agency and action, will be correspondingly contentious and vague. Borderline cases may, in the end, generate problems for the theory of ability. But such problems will not be central here. For