I’m interested in looking at the plausibility of using counterpossibles – subjunctive conditionals with impossible antecedents – in philosophy for a few different reasons. I think we do, in fact, use them (or at least use what we should think are counterpossibles) pretty often. Suppose you’re an eternalist who thinks that eternalism is a necessary truth. If you want to offer an argument against presentism, you may run a quick modus tollens argument like this:
(1) If presentism were true, then some truths would lack truthmakers.
(2) But no truth can lack a truthmaker.
(3) Therefore, presentism cannot be true.
Now you may think this is a bad argument against presentism, but, of course, that’s not important for my point. Another worry is that the first premise of an argument like this isn’t usually put as a subjunctive conditional. But I don’t really know enough about the issue to know how important that is (hence the post).
Anyway, the point of this is to show that, if you’re an eternalist who thinks that eternalism is a necessary truth, then you should think that (1) is a counterpossible. The standard treatment of counterpossibles (following along with the standard Lewisian treatment for counterfactuals) is that they are vacuously true: “If it were the case that p, then it would have been the case that q” is true just in case the closest p-world is also a q-world (there are counterexamples to this as stated, but it will do for now). But if you want to run the above argument against presentism, then you shouldn’t think that (1) is vacuously true – you should think it makes a substantive claim. So if you want to use arguments like the one above in metaphysics (or any other discipline where many of the theories are taken to be necessary truths), then you should think that counterpossibles aren’t vacuously true.
From the little bit that I’ve read, I’ve also seen people make this point (Brogaard and Salerno cite Nolan). There seems to be a significant difference between (a) and (b):
(a) If Goedel had proved that arithmetic is complete, then Hilbert’s program may have been successful.
(b) If Goedel had proved that arithmetic is complete, then Goedel wouldn’t have proved that arithmetic is complete.
(a) sounds true, while (b) sounds false. But if counterpossibles are vacuously true, then (a) and (b) are both (vacuously) true, since the antecedent is impossible.
I think that there may be interesting applications for counterpossibles in philosophy, even beyond running modus tollens arguments against our opponents, as long as we do not treat them as vacuously true. I know that Brogaard and Salerno have a couple of papers on them, and Nolan has written something as well. I suppose I should also have a look at Lewis’s Counterfactuals. Any other suggestions?