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In this essay, building on the work of Robert Stalnaker and David Lewis, I sketch a theory describing the context-dependence of certain modal sentences , including counterfactual sentences. Then, I reveal its potential by briefly considering its application to a familiar argument for fatalism and a recent exchange about time-traveler freedom between Kadri Vihvelin and Ted Sider. My discussion provides a new take on the flaws and the seductiveness of both the fatalist argument and the freedom paradox, a take that may even have application to arguments for incompatibilism advanced by Carl Ginet and Peter van Inwagen. Stalnaker on Context, Lewis on Boundary According to Stalnaker, context includes information presumed to be shared by the participants in the conversation. He proposes representing this information, the common ground, by a set of possible worlds, the context set. Intuitively, the presumed-to-be-shared information is what is true in all the members of the context set. Stalnaker takes this information to be what is presupposed. He also takes the common ground to play an important role regarding assertion. If I utter ‘The king of France is bald’, then, in order for me to assert that the king of France is bald, the common ground must include the presupposition of that sentence, the proposition that there is presently one and only one king of France. So, for Stalnaker, the context set must include only worlds in which there is just one such king. It is not required that the king of France be bald in these worlds. Indeed, that would undermine the point of the assertion. “Assertions . . . are proposals to change the context by adding the information that is their content ” (Stalnaker 1999, 111). I will adopt Stalnaker’s account of context as just presented with two minor modifications. First, instead of representing the common ground as 3 Context, Conditionals, Fatalism, Time Travel, and Freedom John W. Carroll 80 J. W. Carroll a set of possible worlds, I will represent it as a set of propositions. Hence, content is added to the common ground by adding a proposition rather than by eliminating possible worlds. This avoids the consequence that every proposition entailed by a presupposition is thereby a presupposition. Second, in the representation of the common ground, it will be useful to keep track of which of the propositions are suppositions and which are presuppositions . Both presuppositions and suppositions can be bits of presumed -to-be-shared information, but, as I see it, presupposing P includes a commitment to the truth of P that supposing P does not. As a result, and as I will illustrate below, important features of modal utterances are sensitive to the falsity of a presupposition in a manner that they are not sensitive to the falsity of a supposition. Just so, it will be helpful that I not represent the common ground (as Stalnaker does) in one uniform presuppositional lump. In “Scorekeeping in a Language Game,” Lewis suggests that, for an utterance of a sentence with modal terms, context determines the sentence’s truth conditions: The boundary between the relevant possibilities and the ignored ones . . . is a component of conversational score, which enters into the truth conditions of sentences with ‘can’ or ‘must’ or other modal verbs. (Lewis 1983, 246) My favorite illustration of this idea is from “The Paradoxes of Time Travel”: To say that something can happen means that its happening is compossible with certain facts. Which facts? That is determined, but sometimes not determined well enough, by context. An ape can’t speak a human language—say, Finnish—but I can. Facts about the anatomy and operation of the ape’s larynx and nervous system are not compossible with his speaking Finnish. The corresponding facts about my larynx and nervous system are compossible with my speaking Finnish. But don’t take me along to Helsinki as your interpreter: I can’t speak Finnish. My speaking Finnish is compossible with the facts considered so far, but not with further facts about my lack of training. (Lewis 1986, 77) Lewis uses this idea to resolve the grandfather paradox. Suppose Tim’s grandfather died of natural causes in 1957. Still, Tim wishes that he had killed Grandfather. Tim hops into a time machine and emerges in 1920. After careful planning and training, Tim is well prepared to murder his grandfather in 1921. Tim’s killing Grandfather that day in 1921 is compossible with a fairly rich set of facts: the facts about his ri...

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