Here is a picture of how choices work. Suppose a binary choice between A and B, and suppose what you choose is A. Then there are two events. There is a token of the event type c(A,B) of your choosing between A and B, and there is the token of the event type w(A) of your willing A (or of your "being set" for A). What makes it true to say that you chose A over B? It is not simply that a c(A,B) event happens and then a w(A) event happens. For instance, that a c(A,B) event happens and then a w(A) event happens happens is compatible with the hypothesis that first you're choosing between A and B, and then something external causes you to interrupt that process of choosing and forget all about B, and instead you embark on a process of choosing between A and C, and you choose A over C and hence a w(A) event happens. Nor is it true that you chose A over B provided that the w(A) event follows right after the c(A,B) event. For it could be that some external cause causes the w(A) event while suspending the causal effects of the c(A,B) event. Then you didn't choose A over B, because the choosing between A and B was causally irrelevant to your choosing A.
So, maybe we should rather say this: What makes it true to say that you chose A over B is that a w(A) event is caused by a c(A,B) event. That is close, but not quite right. After all, there could just be some weird causal chain on which a c(A,B) event causes an external cause to cause w(A), and that won't be a case of choosing A over B. We need to say:
- CHOICE: What makes it true to say that you chose A over B is that a w(A) event is caused by a c(A,B) event in the right way.
Notice that in the above I said nothing about whether the causation between the c(A,B) event and the w(A) event is deterministic or not. The above story about choosing is one that both compatibilists and libertarians can adopt, though they will likely spell out "in the right way" differently. It is likely that libertarians will understand choice in such a way that the "in the right way" condition requires, among other things, that the causation be indeterministic.
So what's the point of this? Here is one point. Frankfurt proposes as a counterexample to the principle that if you are freely doing something then you could have done otherwise a case where a neurosurgeon watches what you're about to do. If you're about to do B, then he intervenes and makes you do A instead. But if you're going to do A, he doesn't intervene. You do A, and so he doesn't intervene. Surely you're free, but you couldn't have done otherwise.
Fair enough. But the libertarian really cares more about choices here. It is the choice in which you acquire primary responsibility. So, the libertarian can very reasonably retreat to:
- PAPC: If you freely chose A over B, then you could have failed to choose A over B.
But it is far from clear that the neurosurgeon case challenges PAPC in a way that does not beg the question against the libertarian. For the neurosurgeon would have to be able to ensure you to choose A over B, if you were about to choose B over A, in order for the story to work. But how could he do that?
One option would be to modify the c(A,B) event so that it deterministically causes a w(A) event. But the libertarian can reasonably say that a part of the "in the right way" condition in CHOICE is that w(A) is caused indeterministically by c(A,B). The other way would be to produce w(A) by means of some external cause, but then the w(A) event either won't be caused by the c(A,B) event or won't be caused by it in the right way, and so the agent won't be choosing A over B.
These considerations make it plausible that on the libertarian's view, no one can ensure you will choose A over B. And of course the claim that no one can ensure you will choose A over B is incompatible with determinism (since if determinism is true, someone could set up conditions before your conception such that you would have to choose A over B). And this, in turn, suggests that the libertarian can afford to say that to freely choose A over B is just to choose A over B.
I suspect that my above remarks are partly inspired by Richard Gale telling me that he thought Frankfurt and Locke showed you couldn't act otherwise but not that you couldn't choose otherwise. But I didn't manage in the above to show that if you freely chose A over B, you could have chosen B over A.