n 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 (MA) is concerned with modality in a stricter sense: she proposes that ability may be analyzed in terms of the precise framework of propositions and possible worlds just adumbrated. The opponent of (MA), in turn, grants that ability has something to do with possibility but denies that any such analysis succeeds.
Second, while (MA) has been presented as an alternative to (CA), (CA) is arguably just one particular version of (MA). For, as noted above, (CA) appeals to a subjunctive conditional, and the standard semantics for the subjunctive conditional (developed in slightly different ways in Stalnaker 1968 and Lewis 1973) is told in terms of quantification over possible worlds. Specifically (on Stalnaker’s version) a subjunctive conditional is true just in case its consequent is true at the world where its antecedent is true that is otherwise maximally similar to the actual world. In these terms, (CA) is roughly equivalent to the following:
(CAmodal) S has the ability to A iff S does A at a world at which S tries to A that is otherwise maximally similar to the actual world.
This is patently a version of (MA), with ‘S tries to A and is otherwise maximally similar to the actual world’ serving as condition C.
If this is correct, then most discussions of the analysis on ability in the twentieth century have focused on a special and somewhat idiosyncratic case of a much broader program of analysis, namely the program of giving a modal analysis of ability. The considerations brought forth in the remainder of this section, in contrast, are concerned with the general case.
There are two questions that might be raised for this proposal about ability. First, does ability indeed admit of some kind of modal analysis? Second, if it does, how exactly are we to spell out the details of that analysis — in particular, how are we to articulate condition C? Let us begin with the first, more basic, question.
According to (MA), performing an act at some possible world (or set of worlds) is both necessary and sufficient for having the ab