e one in the relevant sense. Yet such an agent lacks the ability to eat a red candy, in precisely the same way as she does in Lehrer’s original example.
It is an interesting question how we might develop other ‘supplementation’ strategies for (CA) (such strategies are also suggested by Ginet 1980), though the track record of this method of analysis in other domains (for instance, the project of ‘supplementing’ the analysis of knowledge in terms of justified true belief, in response to (Gettier 1963)) does not inspire confidence.
There is a surprising disconnect between the way abilities have been discussed in the philosophical literature in the tradition of Hume and the way that they have been approached in more recent work in logic and linguistics. Here, ability claims are understood as categorical possibility claims: claims about what some agent does in some non-actual state of affairs (or ‘possible world’). Let a modal theory of ability be any theory on which claims about an agent’s abilities are understood in terms of claims about what that agent in fact does at some possible world (or set of worlds). The idea that some such modal theory of ability must be true is a presumption of much formal work on ability and ability-ascriptions. Yet there are serious challenges to the idea that ability is in this sense a modality.
Intuitively, claims about ability are claims about possibility. It is in some sense a truism that someone is able to perform some act just in case it is possible for her to perform it.
To develop this purported truism, begin with the thought that for S to have an ability to A it is necessary, but not sufficient, that it be possible that S does A. This claim will be contentious for various more specialized sorts of possibility, such as nomic possibility. But if we may help ourselves to the idea of possibility simpliciter (‘metaphysical possibility,’ on at least one reading of that phrase), then this claim appears plausible. (We will survey some historical and contemporary challenges to it below in Section 4.2.) On the other hand it seems implausible that this sort of possibility is a sufficient condition: there are any number of acts that that are metaphysically possible to perform that an agent might nonetheless not be able to perform.
This suggests a natural hypothesis. To have an ability is for it to be possible to A in some restricted sense of possibility. As nomic possibility is possibility relative to the laws of nature, and epistemic possibility is possibility relative to what an agent knows, so may ability be possibility relative to some independently specifiable set of conditions.
To render this hypothesis precisely, we may help ourselves to the formal framework of ‘possible worlds,’ which offers an elegant and powerful semantics for modal language. On this framework, a proposition is possible just in case it is true at some possible world. We can then say an agent is able to A just in case she performs that act at some world (or set of worlds) that satisfy some independently specifiable set of conditions.
We thus arrive at the modal analysis of ability:
(MA) S has the ability to A iff S does A at some world (or set of worlds) satisfying condition C.
(MA) is actually not itself an analysis but rather a template for a general family of analyses. Different members of this family will be distinguished by the different candidates they might propose for C, as well as whether they quantify over individual worlds or sets of worlds. Nonetheless, these analyses demonstrate sufficient theoretical unity that they may be viewed, at an appropriate level of abstraction, as a single approach to the analysis of ability.
Two points about (MA) bear noting. First, ‘modal’ here is being used in a relatively strict and narrow way. Sometimes ‘modal’ is used loosely to describe phenomena that are connected to possibility (and necessity) in some way or other. As noted above, it is a truism that there is some connection between ability and necessity, and so that ability is in this loose sense ‘modal.’ The proponent of (