PAP vs. PPP

Frankfurt's goal was to argue that determinism and moral responsibility are compatible.  He attempted to prove this by showing that PAP is false.  PAP is the principle that you are morally responsible for your actions only if you could have done otherwise.  PAP is a classic way of stating that incompatibalist thesis.  In other words, PAP means that moral responsibility and determinism are mutually exclusive.  By using thought experiments (Jones and Black) to show that PAP is false, Frankfurt attempted to defend the thesis of compatibalism: determinism and moral responsibility are compatible.

Van Inwagen thinks that Frankfurt has not successfully argued for the compatibalist thesis.  Van Inwagen thinks that PAP can be replaced with a better principle: PPP.  The Principle of Possible Prevention states that you are morally responsible for a state of affairs only if you could have prevented it from obtaining.  In other words, you are morally responsible for something only when you could have prevented that thing from happening.  PPP has the benefit that it cannot be proven false by thought experiments such as Jones and Black.