The dilemma is constructed around finding a story that handles the interaction between modals and conditionals. The prima facie data is that the following pairs are equivalent:
- If p, must be q
- If p, q
- If p, might be q
- Might be (p&q)
It's a really familiar tactic, when presented with a putative equivalence that causes trouble for your favourite theory, to say that the pairs aren't equivalent at all, but can be "reasonably inferred" from each other (think of various ways of explaining away "or-to-if" inferences). But taken cold such pragmatic explanations can look a bit ad hoc.
So it'd be nice if we could find independent motivation for the inequivalence we need. In a related setting, Bob Stalnaker uses the acceptability of Moorean-patterns to do this job. To me, the Stalnaker point seems to bear directly on the Gillies dilemma above.
Before we even consider conditionals, notice that "p but it might be that not p" sounds terrible. Attractive story: this is because you shouldn't assert something unless you know it to be true; and to say that p might not be the case is (inter alia) to deny you know it. One way of bringing out the pretty obviously pragmatic nature of the tension in uttering the conjunction here is to note that asserting the following sort of thing looks much much better:
- it might be that not p; but I believe that p
(I can sometimes still hear a little tension in the example: what are you doing believing that you'll catch the train if you know you might not? But for me this goes away if we replace "I believe that" with "I'm confident that" (which still, in vanilla cases, gives you Moorean phenomena). I think in the examples to be given below, residual tension can be eliminated in the same way. The folks who work on norms of assertion I'm sure have explored this sort of territory lots.)
That's the prototypical case. Let's move on to examples where there are more moving parts. David Lewis famously alleged that the following pair are equivalent:
- it's not the case that: if were the case that p, it would have been that q
- if were that p, it might have been that ~q
- if it were that p, it might have been that not q; but I believe if it were that p it would have been that q.
We find pretty much the same cases for "must" and indicative "if".
- It's not true that if p, then it must be that q; but I believe that if p, q.
These sorts of patterns make me very suspicious of claims that "if p, must q" and "if p, q" are equivalent, just as the analogous patterns make me suspicious of the Lewis idea that "if p, might ~q" and "if p, q" are contradictories when the "if" is subjunctive. So I'm thinking the horns of Gillies' dilemma aren't equal: denying the must conditional/bare conditional equivalence is independently motivated.
None of this is meant to undermine the positive theory that Thony Gillies is presenting in the paper: his way of accounting for lots of the data looks super-interesting, and I've got no reason to suppose his positive story won't have a story about everything I've said here. I'm just wondering whether the dilemma that frames the debate should suck us in.