Protagoras of Abdera was one of several fifth century Greek thinkers (including also Gorgias, Hippias, and Prodicus) collectively known as the Older Sophists, a group of traveling teachers or intellectuals who were experts in rhetoric (the science of oratory) and related subjects. Protagoras is known primarily for three claims (1) that man is the measure of all things (which is often interpreted as a sort of radical relativism) (2) that he could make the "worse (or weaker) argument appear the better (or stronger)" and (3) that one could not tell if the gods existed or not. While some ancient sources claim that these positions led to his having been tried for impiety in Athens and his books burned, these stories may well have been later legends. Protagoras' notion that judgments and knowledge are in some way relative to the person judging or knowing has been very influential, and is still widely discussed in contemporary philosophy.
Protagoras' doctrines can be divided into three groups:
- Orthoepeia: the study of the correct use of words
- Man-measure statement: the notion that knowledge is relative to the knower
- Agnosticism: the claim that we cannot know anything about the gods