The traditional debate between compatibilists and incompatibilists was based on the assumption that if determinism deprives us of free will and moral responsibility, it does so by making it true that we can never do other than what we actually do. All parties to the debate took for granted the truth of a claim now widely known as "the principle of alternate possibilities": someone is morally responsible only if he could have done otherwise. In a famous paper, Harry Frankfurt argued (...) that the principle of alternate possibilities is false. I argue that Frankfurt's argument rests on a modal fallacy. (shrink)