In recent years, some philosophers have claimed that we can know a priori that certain external world skeptical hypotheses are false on the basis of a priori knowledge that we are in certain kinds of mental states, and a priori knowledge that those mental states are individuated by contingent environmental factors. Appealing to a distinction between weak and strong a priority, I argue that weakly a priori arguments of this sort would beg the question of whether the skeptical hypothesis under assessment is true, and that the prospect of a sound strongly a priori argument of this sort seems dim.

‘It still remains a scandal to philosophy... that the existence of things outside of us... must be accepted merely on faith, and that, if anyone thinks good to doubt their existence, we are unable to counter his doubts by any satisfactory proof.’

(Immanuel Kant, Critique of Pure Reason)