This thesis is primarily focussed on developing a novel characterisation of the distinction between a priori and a posteriori justification. My working hypothesis is that we can make a surprising amount of progress in this field by paying attention to the structure of epistemic norms. I argue that direct a priori beliefs are governed by a structurally different kind of epistemic norm to the one that governs perceptual beliefs. That, I argue, is where the fundamental epistemological difference between the two categories lies. I call this view ‘Seeming-Independence’. Seeming-Independence holds that while a posteriori beliefs depend epistemically on how it perceptually seems to us, there is no corresponding dependence relation between a p...