1 Counterpossibles in Metaphysics [to appear in Brad Armour-Garb and Fred Kroon (eds.), Philosophical Fictionalism; draft of 12 September 2016] Timothy Williamson If whales were fish, their behaviour would differ from what it actually is. If whales were fish, their behaviour would be just as it actually is. Those sound like genuine alternatives. Yet, since whales are by nature mammals, they presumably could not have been fish; that would be contrary to their nature. Thus both conditionals are counterpossibles, counterfactual conditionals with impossible antecedents. Semantic orthodoxy makes all counterpossibles true. So the two conditionals were true, and not mutually exclusive after all. Is orthodoxy about counterpossibles correct? The problem is not just how best to tidy up an unimportant little corner of the logic and semantics of counterfactuals. It has significant theoretical and methodological ramifications in several directions. This paper defends orthodoxy against recent objections, and explains recalcitrantly unorthodox appearances by our pre-reflective reliance on a fallible heuristic in assessing conditionals. 1. What is at stake A counterfactual is a conditional sentence like the so-Đalled suďjuŶĐtiǀe If this ǁeƌe so, that ǁould ďe so aŶd uŶlike the indicatiǀe If this is so, that is so. Typically, we use counterfactuals to talk about what would have happened if something had been different from how it actually was. Still, despite the etymology, a counterfactual may have a true aŶteĐedeŶt; If she ǁeƌe depƌessed, that ǁould edžplaiŶ heƌ sileŶĐe does Ŷot iŵplLJ that she is not depressed. But a counterpossible is a counterfactual whose antecedent is impossible, so false. What kind of impossibility is relevant? It is not epistemic. For consider this counterfactual: (1) If thinking had never occurred, science would have flourished. The antecedent of (1) is epistemically impossible, because it is incompatible with something we know: that we think. But that does not make the antecedent of (1) impossible in the relevant sense. Presumably, the universe could have been lifeless and thoughtless forever.