 By ‘a priori knowledge’ Warfield means knowledge that is obtained without either perceptual observation or empirical investigation (1998: 80; see also McKinsey 1991). Warfield’s view then is that one can always have privileged a priori knowledge of the propositional contents of one’s thoughts (the traditional principle of privileged access), even when these contents are external or wide, in the sense that having a thought with that content logically implies the existence of particular objects or substances that are external to the agent (see the useful entry by Gertler 2017 on  self-knowledge).