 However, this contention that one and the same proposition can be both a posteriori and necessary is considered absurd by some philosophers (as is Kripke's paired claim that the same proposition can be both a priori and contingent).For example, Robert Stalnaker's account of knowledge represents knowledge as a relation on possible worlds, which entails that it is impossible for a proposition to fail to be a priori given that it is necessary.This can be proven as follows