The Principle of Alternate Possibility says something like:
- (PAP) If x freely does A, then x is able to do otherwise than A.
- Were x to try in circumstances C, x would do A
- (DispAb) x is able to do B in circumstances C if and only if x has a disposition to do B when trying to do B in C.
Very neat, except that it runs into one serious difficulty. The definition of ability cannot be plugged into PAP. To plug it into PAP, we would need DispAb to define being able to do B. But DispAb doesn't define that. It defines being able to do B in circumstances C. And PAP has no mention of circumstances. In other words, PAP uses a two-place concept of ability—x is able to do A—while DispAb defines a three-place concept—x is able to do A in C.
Maybe this is much ado about nothing. While PAP doesn't mention circumstances, we should take it to say:
- (PAP') If x freely does (variant: chooses) A, then x is able to do (variant: choose) otherwise than A in these circumstances.
- The circumstances of deciding between A and B while there is a neurosurgeon who upon observing that you are about to try to do otherwise than A will prevent you from doing otherwise than A.
Of course, one might use coarser-grained circumstances:
- The circumstances of deciding between A and B.
The above approach to figuring out what the relevant circumstances are is too ad hoc anyway. Obviously, one can specify the circumstances at varying levels of descriptive detail. We need a principled way to decide how much detail. The minimal level of detail is given by (2). We have already seen that that's too little. We presumably cannot include in the circumstances what the decision itself is—we get irrelevantly weird stuff if we ask what you are disposed to do in the odd circumstances of trying to do B when having decided to A. So that would be too much. Where can a principled line be drawn? I think the three most natural non-arbitrary options are:
- The circumstances are all events that are causally prior to your decision.
- The circumstances are all events that are not your decision nor causally posterior to your decision.
- The circumstances are the complete state of the universe temporally just prior to your decision.
But now notice that given causal determinism, none of these three ways of specifying the circumstances is going to allow you to maintain PAP in a causally deterministic world. For each of them in a deterministic universe is sufficient, at least in conjunction with the laws[note 1], to fix what you're going to decide.
So all the non-arbitrary ways I can think of for spelling out the circumstances to plug the dispositional account of ability into PAP are not ones a compatibilist who wants PAP to hold can embrace.
But interestingly Fara's dispositional approach can help the incompatibilist! For those of us who think that causal (or at least explanatory) priority is the most important thing vis-a-vis freedom will, I think, be drawn to (3) as the right description of the circumstances. But the libertarian can say that in the neurosurgeon-type Frankfurt case, you do have the disposition to act otherwise than A when trying to do A in circumstances C satisfying (3). For the neurosurgeon's activities are not causally prior to your decision. (And if they were, then the Frankfurt case would make your action be determined by causal factors prior to your action, and that would beg the question against the typical libertarian.)
So, to recap, a non ad hoc filling out of the details in the dispositional account of ability (a) is incompatible with saving PAP on compatibilism but (b) can help the libertarian with Frankfurt examples.