The standard formulation of relativism in ancient philosophy was the homo-mensura statement of Protagoras:
"Man is the measure of all things, of those that are that they are and of those that are not that they are not."
Plato's dialogue, Theaetetus, is an extended refutation of the homo-mensura doctrine and its epistemological consequences. First, he argues that the individual judgement can be deranged. If a person has a fever, he may perceive the temperature as hot that an unfevered man would perceive as cold. But it cannot be hot and cold, and thus we cannot accept as the criterion the judgement of the individual, i.e. the possibility of individuals having distorted judgements precludes the possibility that each thing is as the individual judges it. Next, in the dialogue, the discussion moves to a refutation of knowledge as perception. Then it argues that since the object of knowledge must remain stable the Heraclitean theory of flux implies that perception can only lead to opinion not knowledge.