PROTAGORAS OF ABDERA

(c 480-410 BC) All mental activity consists in, or is reducible to, perception s, which are the product of the motion of the knower and the motion of the things known. What is known is sense data, not independent objects. What is known is relative to the knower and to the instant of perception . Hence "man is the measure of all things, of things that are that they are and of things that are not that they are not." Secondary qualities (e.g., color, taste) are subjective.