Note (return) — Chapter VIII

The psychological opinions of Democritus were as decidedly materialistic as his physical theories. All knowledge is derived from sensation. It is only by material impact that we can know the external world, and every sense is, in reality, a kind of touch. Material images are being continually thrown off from the surface of external objects which come into actual contact with the organs of sense. The primary qualities of matter, that is, those which are involved in extension in space, are the only objects of real knowledge; the secondary qualities of matter, as softness, hardness, sweetness, bitterness, and the like, are but modifications of the human sensibilities. "The sweet exists only in form--the bitter in form, hot in form, color in form; but in causal reality only atoms and space exist. The sensible things which are supposed by opinion to exist have no real existence, but atoms and space alone exist." 426