As democracy flourished in fifth-century BCE Athens, the art of persuasion — especially speaking well in the assembly and the court — became an instrument of power. The travelling teachers who met this need were called "Sophists" (masters of wisdom); for a fee they taught rhetoric, politics, and the art of argument. The most famous was Protagoras of Abdera (c. 490–420 BCE).

The famous saying attributed to Protagoras divides the history of philosophy: "Man is the measure of all things — of the things that are, that they are, and of the things that are not, that they are not." This sentence declares a radical relativism: there is no objective truth valid for everyone; a thing is for me as it appears to me, and for you as it appears to you. A wind may feel cold to one and warm to another — and neither is mistaken. Truth depends on the measuring subject.

The Sophists carried this view into ethics and society: justice, custom, and laws are not divine or natural but human-made conventions that vary with society. This was a bold questioning of religious and traditional authority; but it also carried the danger, "if every view is equally true, is there any such thing as truth?"

It was precisely against this relativism that Socrates and Plato arose. Plato portrayed the Sophists as people who cared not for truth but for winning, who "make the weaker argument appear the stronger" — which is why the word "sophist" carries a negative sense today. But the Sophists' question never closed: is truth absolute, or does it vary with the human and the culture? That question remained at the centre of philosophy, from ancient skepticism to modern debates on relativism and cultural pluralism.