[zenogais]

He's getting that argument from good ole Kant and his Critique of Pure Reason from 1781 (http://en.wikipedia.org/wiki/Critique_of_Pure_Reason). The basic argument, so far as I understand it, is that there must exist certain "structures in the brain" that make all our understanding possible. Since these are brain structures they must be a priori (everyone is born with them). On the side of aesthetic senses he boils these down to space and time. From our sense of space it is possible for us to reason about geometry and therefore much more abstract forms of mathematics without having to have recourse to the "outside" world. So we can envision and prove new geometrical proofs and our internal brain structures for understanding space ground these proofs in a sort of sense certainty. To go to your example of 2 + 2 we can say that nowhere in these symbols is contained the meaning 4 - so no amount of analysis (dividing these symbols up, reducing them to more primitive symbols, etc) will get us to the equality - we have to perform some operation internally and that operation has to be grounded on something a priori - these would be our sense of space where we can envision two things and two more things and perform the count and arrive at 4 - all done outside of experience.

Now there are plenty of philosophers who came later who have tried instead - following in the shoes of Hume - to perform a materialist grounding of these faculties not in the brain but derived from sense experience itself - sense experience somehow constitutes the structures of the brain and changing external sense experiences can reshape or reconstitute these brain structures. For example, Gilles Deleuze seems to me to do something like this.