在討論自由意志問題中，我們要問：為什麼我們應該關心自己是否有自由意志以及決定論是否是真的？我們之所以關心自己是否有自由意志是因為我們關心道德責任。一個行為者如果沒有自由意志，那麼就不能要求他負道德責任。這個原則就是其他選擇可能性原則”(the principle of alternative possibilities 簡稱PAP) 。PAP 是這樣的：

There is a important principle in the problem of free-will. This principle is called “the principle of alternate possibilities” ( hereinafter : PAP)which states that a person is morally responsible for what he has done only if he could have done otherwise. Harry G. Frankfurt has presented a series of putative counterexamples to PAP. (“Alternate Possibilities and Moral Responsibility” Journal of Philosophy 66,1969,pp.829-39)