[P1] holds that a non-introspective belief that p is justified only if the proposition believed plays a role in the best explanation of S's mental states.

Even if there were sufficient intuitive support for [P1], defending this argument by such means would contravene both strands of the non-self-undermining constraint (Pust 2001). According to [P1], S’s non-introspective belief that p is justified only if the proposition believed plays a role in the best explanation of S’s mental states. No demonstration of explanatory relevance is involved in the two methods of justifying [P1] just discussed. Rather, each approach would take the mere fact that th