Section 1. "it Is to Be Had Only by Sensation."

Section 2. "Instance whiteness of this paper."

Section 3. "This, though not so certain as demonstration, yet may be called 'Knowledge,' and proves the existence of things without us."

Locke's argument proceeds, as we have seen, on the assumption that we perceive external things directly,--an assumption into which he slips unawares,--and yet he cannot allow that we really do perceive directly what is external. This makes him uncomfortably conscious that he has not absolute proof, after all. The section that closes the discussion is entitled: "Folly to expect demonstration in everything."