First, we have to come up with an account of responsiblity that accounts for all the times when we absolve people of responsiblity on the grounds that they couldn't help it. On the face of things, every time we tell a teacher we have good reason not to be penalized for turning a paper in late, we are appealing to the principle of alternate possibilities. So before I start worrying about Frankfurt cases, I want to know what the opponent of PAP's story is on normal, everyday cases. Why does it suffice to say that I couldn't attend class because I was in the hospital and couldn't get out in time to attend? Why don't we say "That's still your fault. Just because you couldn't have attended class because you were in the hospital doesn't mean that you are free of responsiblity for showing up for class?" When people make excuses for failure to perform, we may in fact think that they aren't telling is the truth, or, at least, the whole story. But if Flip Wilson's Geraldine is right that the devil made her buy the dress, then we'd have to say she's not responsible. Otherwise, she wouldn't be even bothering to use that as an excuse. I'd like to tell the compatiblist "You explain our ordinary excuse-making and excuse evaluating practices, before I have to explain what is supposed to be happening in Frankfurt cases." I'm not saying this can't be done; I am saying we need to deal with normal cases before we deal with Frankfurt cases.
Second, I take it that one critical element of a responsible choice is deliberation. It at least has to be possible for the agent to deliberate. Indeed there is a class of actions which are rightly criticized because the agent didn't deliberate before acting, so there is a kind of sub-choice as to whether or not to deliberate. The Murder 1/ Murder 2 distinction suggests that we don't accord the highest level of moral responsibility in the absence of deliberation.
Which brings us to our Frankfurt cases. Let's say the Devil is our controller. Tiger, a married man, is being tempted to commit adultery with a cocktail waitress named Jaimee. As it happens, the Devil intends to make Tiger commit adultery unless he chooses freely to do so. He deliberates on the possibility of committing adultery, and then what? The moment he starts to take the possibility of not committing adultery seriously in the course of his deliberations, the Devil steps in and makes him do it? But the deliberation and serious consideration of the alternative is what might have been required to make him responsible in the first place, or at least fully responsible. So in order stay in keeping with the concept of responsibility embedded in the murder 1-2 distinction, the Devil can't step in until his adulterous action is fully premeditated. So that means the Devil can only step in and make him do it once he begins the process of choosing to refrain from the adultery. But if that has happened, he has already performed an alternative act of will. PAP holds, and the counterexample folds.
Which means, that if Tiger ever figures out about what the Devil was doing, he can't say "The Devil made me do it" unless he starts to refuse, and is then forced to commit adultery even though he was beginning to choose not to commit adultery. If he commits adultery without Satanic assistance, he is responsible because he could have made an alternative choice. The fact that the Devil would have forced the opposite action doesn't meant the choice wasn't possible. If the Devil is an all-determining deity, then we can all say "The Devil Made Me Do It." Ditto for the Calvinistic God.