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 ability to perform that act. One way of challenging this claim is to deny the necessity claim: that is, to argue that it is sometimes the case that an agent is able to perform an act that she does not perform at any possible world.
This is an argument that has in fact been made by several authors. Descartes, for one, appears to have argued that God is such an agent (Curley 1984). A genuinely omnipotent being, one might argue, should be able to perform any act whatsoever, even the impossible ones. This view of omnipotence is contentious, but it is not clear that it should be ruled out formally, by the very analysis of ability, as (MA) does. Spencer (2017) argues that even non-omnipotent agents may sometimes have the ability to perform acts that they do not perform at any possible world.
Let us grant, however, that the possibility of performing an act is a necessary condition for performing that act, and that in this sense an attribution of ability entails a possibility claim. One might nonetheless resist the view that ability admits of a modal analysis in the manner suggested by (MA).
That is the kind of argument developed in a prescient discussion by Anthony Kenny (Kenny 1975; the presentation of Kenny here is indebted to the discussion in Brown 1988). Kenny argues that, if something like (MA) is indeed true, then ability should obey the principles that govern the possibility operator in standard modal logics. Kenny claims that ability fails to satisfy the following two principles:
(1) \(A \to \Diamond A.\)
Informally, (1) expresses the principle that if an agent performs an action, then she has the ability to perform this action. This is, Kenny argues, false of ability.
(2) \(\Diamond(A \lor B) \to (\Diamond A \lor \Diamond B).\)
Informally, (2) expresses the principle that if an agent has the ability to perform one of two actions, then she has the ability to perform either the first action or the second action. This is, Kenny argues, false of ability.
Let us begin with (1). Kenny claims that this principle is fal