Reason and preference
(Note: In one of those annoying moments of (dis?)sycrhonicity I've finished up this post just as Alex has posted his piece below - make sure you don't overlook it! It's not as though I'm that desperate for attention that I need to try and overshadow other people)
According to many people in the field, modern philosophy starts with Rene Descartes, or more specifically with one of the most famous phrases in the history of philosophy: "I think, therefore I am."
Descartes hoped to establish the foundational beliefs of human understanding – to find that which could not be doubted without contradiction. To do this he set aside millennia of accumulated human knowledge and went back to (what he believed to be) the central, unquestionable, truth at the heart of all experience.
Only that which exists can think.
Therefore, I exist.
However, this is not the fundamental truth that Descartes supposed it to be – it actually presupposes two, quite crucial, things:
Without reason, no argument can be made – or at least no argument worth making.
Without preferences (with which I include desire, value, etc.) there can be no motivation for making an argument.
Descarte (and all sceptics) must assume not only the existence of these two things, but also their validity – as otherwise there are no grounds for making any argument. Without reason, we have no means to, without preference we have no grounds to. This holds regardless of our metaphysics – as metaphysics itself is impossible without them. They are fundamental aspects of our existence and as such largely beyond analysis – once we start to question them our means to and grounds for doing so are pulled from under us: If we question reason, then we cannot construct a rational argument against it. If we question preference, then we have no grounds for preferring it to be valid.