Abstract: Bill Brewer explores a solution to a reductio ad absurdum argument template, one instance of which has the conclusion that we can know the proposition ‘someone has interacted with water’ a priori. Brewer holds that empirical knowledge of water is required for a thinker to possess the concept ‘water’ in the first place, so this knowledge cannot be said to be derived a priori by means of the argument.

Keywords: a priori knowledge, Bill Brewer, empirical knowledge, externalism, Reductio ad absurdum,