degree of uniformity from the other, and that one affords no inference by which we can conclude the existence of the other. For these are plain and acknowledged matters of fact. By liberty, then, we can only mean a power of acting or not acting, according to the determinations of the will; this is, if we choose to remain at rest, we may; if we choose to move, we also may. Now this hypothetical liberty is universally allowed to belong to every one who is not a prisoner and in chains. (8.1; Hume 1748, 72)
Of course, Hume and many of those who have followed him have been attempting to do something rather more than to offer a theory of ability. Hume’s intent was to show that disputes over ‘question of liberty and necessity, the most contentious question of metaphysics’ have been ‘merely verbal’ (8.1; Hume 1748, 72). Whatever we may think of this striking claim, however, there is a dialectical gap between it and the alleged truth of (CA). To anticipate a theme that will be central in what follows, we must be careful to distinguish between, on the one hand, the adequacy of various views of ability and, on the other, the more contentious metaphysical questions about freedom to which they are doubtlessly related. It is the former that will be our concern in this section.
(CA) says that satisfying a certain conditional is both sufficient and necessary for having a certain ability. There are two kinds of counterexamples that may be brought against (CA): counterexamples to its sufficiency, and to its necessity. Let us take these in turn.
Counterexamples to the sufficiency of (CA) have been most prominent in the literature. Informally, they are suggested by the question: ‘but could S try to A?’ There are a variety of ways of translating this rhetorical question into a counterexample. We may distinguish two: global counterexamples, according to which (CA) might always get the facts about ability wrong, and local counterexamples, according to which (CA) might sometimes get the facts about ability wrong.
Begin with global counterexamples. Let us say that determinism is true at our world. Familiar arguments purport to show that, if this is the case, then no one has the ability to do anything, except perhaps for what she actually does (for several developments of such an argument, see van Inwagen 1983, 55–105). But if (CA) is true, then agents would have the ability to perform various actions that they do not actually perform. For it is plausible that the conditionals in terms of which (CA) analyzes ability would still be true in a deterministic world. But then, since it makes false predictions about such a world, which for all we know may be our own, (CA) is false.
The difficulties involved in this sort of counterexample are clear. The proponent of (CA) will reject the arguments for the incompatibility of ability and determinism as unsound. Indeed, it is precisely her thought that such arguments are unsound that has typically led her to take ability to be analyzed in terms like those of (CA). So global counterexamples, while they may be successful, are dialectically ineffective relative to the range of questions that are at issue in the debates over ability.
It seems, however, that we can show that (CA) is false even relative to premises that are shared between various disputants in the free will debates. This is what is shown by local counterexamples to (CA). One such example is given by Keith Lehrer:
Suppose that I am offered a bowl of candy and in the bowl are small round red sugar balls. I do not choose to take one of the red sugar balls because I have a pathological aversion to such candy. (Perhaps they remind me of drops of blood and … ) It is logically consistent to suppose that if I had chosen to take the red sugar ball, I would have taken one, but, not so choosing, I am utterly unable to touch one. (Lehrer 1968, 32)
Such an example shows that (CA) is false without assuming anything contentious in debates over freedom. It turns rather on a simple point: that psychological shortcomings, just as much as external impediments, may u