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Crosslinguistically, expressions of ability exhibit a curious duality of interpretation, in some contexts describing the abilities and potential of an agent, and in others simply describing what the agent did on a particular occasion. In languages that mark grammatical aspect, the alternation extends to abilitative uses of the possibility modal, and is governed by aspectual marking (Bhatt, 1999). For instance, the imperfectively marked French modal pouvoir ('can') in (1) is compatible with a 'pure', potentially-unrealized ability interpretation; by contrast, perfective pouvoir in (2) gives rise to an actuality entailment, requiring the realization of its complement, and seemingly very little else.
(Marja could-imperfective swim across the lake, but she never crossed it.)
2. Marja a pu traverser le lac à la nage, #mais elle ne l'a pas traversé.
(Marja could-perfective swim across the lake, #but she did not cross it.)
Actuality entailments have resisted explanation in a literature which aims to derive them in the composition of modality and aspect, treating ability as a circumstantial possibility operator, and the perfective aspect as imposing temporal boundaries on eventualities in its scope (Hacquard, 2006, a.o.). I propose an account which derives both ability and actuality interpretations from a novel component in the semantics of ability: causal dependence. The main idea is that ability modals describe a complex causal structure, in which the (circumstantial) possibility that an agent S will realize an event A(S) obtains in view of the causal dependence of A(S) on an available action for S. This proposal is partially motivated by philosophical work on ability, which suggests that abilitative possibilities have stronger truth conditions than pure circumstantial possibilities (Kenny, 1976; Brown, 1988).
I develop the argument for a causal analysis by comparing actuality inferences to the interpretation of two other types of complement-taking predicates: implicative verbs (e.g., manage; Karttunen, 1971) and enough and too comparatives (e.g., be fast enough; Meier, 2003). I show that, in both cases, inferences about the actualization of the complement follow from causal dependence relations embedded in the lexical semantics and composition of the verbal predicate, and demonstrate that the aspectual class properties of this causal structure interact with viewpoint aspect to produce inferential contrasts parallel to those in (1)-(2). Pursuing this analysis, the actuality entailments of ability modals result not just from the composition of modality and aspect, but more specifically from the composition of aspect with the specific type of complex causal possibility conveyed by ability predicates.
I formalize causal dependence relations over the structure of a causal model which represents causal connections as directed links in a graphical network (Pearl, 2000; Schulz, 2011; Kaufmann, 2013). In such a model, the felicity conditions imposed by causal necessity/sufficiency presuppositions depend crucially on the discourse background. Grammatical aspect then selects for a particular interpretation of the abilitative causal structure by selecting for a particular type of background. The success of a causal dependence analysis in explaining implicative and actuality inferences lends support to a broader program of semantic investigation, which argues that the types of contrasting dependencies that can be defined over formal models of causation play an important role in the semantic representations and linguistic reasoning associated with both overtly and non-transparently causal language.