What is the epistemic significance of our being unable to convince other people of our beliefs? Or: Does being unable to convince someone that P give me reason to doubt that P?
Let's say that a philosopher deploys all the effort and rhetorical skill he can muster, but is unable to persuade his opponent. Why has he failed to convince? There are two principle reasons I can think of: (1) the philosopher and his opponent do not share the same premises, or (2) the philosopher's opponent is irrational (biased, stupid, crazy, etc.). The problem as I see it is that there seems no way to tell who is in the right. Presumably, neither the philosopher nor his opponent can justify their premises, nor can either one show that he is the rational one and the other irrational (the philosopher could just say that his opponent is crazy, but the opponent could say the same thing of him!).
It's problems like this which move me closer to the uncomfortable possibility that to be in the right is often simply to be in the majority.