A priori Knowledge
A priori knowledge is known through reason alone, whilst sitting in your armchair, for example, the cogito 'I think therefore I am' or 'No object can be green and red all over'. While empiricists don't deny the existence of such knowledge they claim that it is trivial and non substantive (apart from maths) whereas rationalists refute that it is important. 2 Ways in which is it is possible to have a priori knowledge is 1) through deduction e.g. maths and 2) innately e.g. God or the forms.
1. One reason that a priori knowledge is important is because it's immune to sceptical doubt. Descartes' shows this using his 3 waves of doubt, to show that only a priori foundations of the cogito and God's existence are indubitable.
-Hume argues that the cogito is simply analytical and circular, it offers no independent justification as an argument. This is the same for the trademark argument which asserts the existence of God as innate knowledge. A priori knowledge can only be "relations of ideas" and not "matters of fact" - synthetic.
2. Kant claims that the conceptual schemes that we have are synthetic, a priori because they shape our world e.g. Condillac's statue/Filing cabinet analogy - to avoid a "blooming buzzing confusion" (William James)