Take determinism to be the thesis that for any instant, there is exactly one physically possible future (van Inwagen 1983, 3), and understand incompatibilism regarding responsibility to be the view that determinism is incompatible with moral responsibility. Of the many different arguments that have been advanced for this view, the crux of a relatively traditional one is this: If determinism is true, then we lack alternatives. If we lack alternatives, then we can't be morally responsible for any of our behavior. Therefore, if determinism is true, then we can't be morally responsible for any of our behavior. The second premise is a version of the principle of alternate possibilities (PAP): persons are morally responsible for what they have done only if they could have done otherwise. This principle, in conjunction with the assumptions that responsibility requires control, and that this control consists in the freedom to do otherwise, provides the vital bridge from the initial premise to the skeptical conclusion. Some incompatibilists, joining ranks with various compatibilists, have sought to reject this principle by invoking so-called ‘Frankfurt examples.’
Harry Frankfurt and Criticism of the Principle of Alternate Possibilities