mply to have a certain kind of potentiality. Vetter also provides an explicit semantics for agentive modality in terms of the ascription of potentiality (Vetter 2013), which constitutes a systematic alternative to (MA).
Yet another view is suggested by recent work by Helen Steward emphasizing the traditional idea of a ‘two-way power’ (Steward 2012, Steward 2020; see also Alvarez 2013). Steward argues that actions are exercises of two-way powers — ‘powers which an agent can exercise or not at a given moment, even holding all prior conditions at that moment fixed’ (Steward 2020, 345). These are to be contrasted with one-way powers, which manifest whenever their manifestation conditions are realized, as a fragile glass breaks whenever it is struck. Unlike both the new dispositionalists on the one hand and Vetter on the other, Steward sees a fundamental asymmetry between agents, who are bearers of two-way powers, and mere objects, who are are bearers only of one-way powers. It is natural then to think that what it is to have an ability is to have a two-way power of a certain kind, and that a theory of ability should follow from an account of agency and the two-way powers.
We thus have at least three ‘power-based’ views of abilities: the new dispositionalism, Vetter’s potentiality-based view, and Steward’s two-way power-based view. It is instructive to consider how these three accounts differ with respect to two larger philosophical projects.
One is the project of giving an analysis of modal language, including ability-ascriptions, in terms of possible worlds. The new dispositionalist rejects (CA) and (MA) but she is still, in principle, sympathetic with that project, insofar as she allows that disposition ascriptions themselves may be ultimately understood in terms of quantification over possible worlds. This kind of view is explicitly endorsed, for instance, by Smith (2003). In contrast, Vetter is explicitly opposed to this project, while Steward’s view appears neutral on this question.
Another is the project of arguing that agents’ abilities are, or are not, compatible with the possibility of determinism. This is a question we touched on briefly in Section 5.1. As noted there, the new dispositionalists are motivated, in part, by the project of arguing for compatibilism. In contrast, Steward is explicitly concerned to defend incompatibilism. (Vetter appears to be neutral on this question.) The question of how the analysis of ability bears on the free will debates looms over many of these discussions, and we will confront it directly and at some length below in Section 6.
Still other approaches to ability reject (CA) and (MA) while rejecting also the thought that some appeal to power — be it a disposition, a potentiality, or a two-way power — is needed to account for abilities.
One recurrent thought is that, despite the various challenges that have been brought against it, ability is still intimately connected to some kind of relation between an agent’s volitions (tryings, intendings) and her actions. One semantically sophisticated development of this thought is Mandelkern et al, 2017, which proposes to take as basic a notion of ‘practically available actions,’ and to develop in terms of these a semantics for agentive modality, one on which conditionals linking volitions to actions continue to play an essential role. Another approach, in a recent book-length treatment of ability (Jaster 2020), proposes that abilities should be thought of in terms of proportions of cases where an agent successfully does what she intends to do (compare the approach to dispositions advocated in Manley and Wasserman 2008). Both of these approaches are explicit in their denial of (CA), but they share — with the new dispositionalists — the thought that the analysis of ability should somehow appeal to a pattern of dependence between volition and action.
This is not surprising. The conditional analysis is a redoubtable proposal, endorsed by many of the major figures in English-language philosophy. In light of that, it seems reasonable to try to disc