What does man actually know about himself?

What does man actually know about himself? Is he, indeed, ever able to perceive himself completely, as if laid out in a lighted display case? Does nature not conceal most things from him-even concerning his own body-in order to confine and lock him within a proud,deceptive consciousness, aloof from the coils of the bowels, the rapid flow of the bloodstream, and the intricate quivering of the fibers! She threw away the key. -Friedrich Nietzsche, On Truth and Lying in an Extramoral Sense (1873)