The Pythagorean theorem isn't proved or or even checked by measuring right triangles and noticing that a^2 + b^2 = c^2. Is the Pythagorean theorem not knowledge?
It had damn well better be checked that way, because it rests on the assumption of flat space, which may or may not be true. The derivation from the axioms is not checked by empirical data; the axioms themselves are. If you don't check the axioms, you don't have knowledge, you have pretty equations on paper, unconnected to any fact. Pythagoras is just as much empirical knowledge as Einstein; it's just that the axioms are closer to being built-in to the human brain, so you get an illusion of Eternal Obviousness. Try explaining the flat-space axioms to squid...
Here's the new thread for posting quotes, with the usual rules: