In Plato's Theaetetus, Socrates asks Theaetetus the question "What is knowledge?" Theaetetus' answer to this question prompts Socrates to offer a characterization of the philosophical positions of the pre-Socratics Protagoras and Heraclitus, which he then proceeds to attempt to refute.

According to Theaetetus' first attempt to answer Socrates' rather vexing question, "knowledge is nothing else than perception". Perception of an object or an event is knowledge of the object or event. Socrates commends Theaetetus for his efforts, but argues that if one adopts such a position, they must also be lead logically to believe the central assertions of Protagoras and Heraclitus. According to Socrates, Protagoras argued that man is the measure of all things, while Heraclitus held that there is no absolute, there is only flux, becoming. If one believes that knowledge is perception, then every man has knowledge which is based on his own personal perception. Necessarily, then, knowledge is different for everyone. Thus one is lead to the Protagorean position, as individual men are indeed the measure of things, as their perceptions determine knowledge. This Protagorean position leads naturally to the Heraclitean position, because if knowledge is different for everyone, then knowledge is always changing. As knowledge is a representation of reality, this means that reality is always changing, that is, in flux, and thus, there is no absolute. There is only becoming.

Socrates offers several refutations of these positions. His explanation of the way in which the Protagorean position leads to the Heraclitean position itself points to the absurdity of the Protagorean position. Socrates shows that if every man's perception is knowledge, then there is no true knowledge, only opinion. A man may judge a wind to be cold, while another judges the wind not to be cold. If this is the case, then either the wind is both cold and not cold, or knowledge amounts to little more than opinion. After fully articulating the resultant Heraclitean position, however, Socrates comes to his real arguments against the positions.

Socrates provides a counterexample to Theaetetus' assertion that knowledge is perception. According to Socrates, if I hear someone speaking a foreign language that I don't know, I perceive their speech, but I do not understand it. Similarly, if something is written in an alphabet which I don't know, I may see the written words but not understand them. Socrates also argues that the Protagorean position is inherently untenable simply because it cannot, by its own argument, be true, that is, universally correspondent to absolute reality. If Protagoras claims that man is the measure of all things, then someone who asserts that man is not the measure of all things is equally as right as Protagoras. If Protagoras and Heraclitus deny the existence of absolute truth, then the truths which they profess cannot be absolute truth. Socrates also argues that respect for techne, scientific knowledge, also shows the falsity of the Protagorean argument. If someone wants to know how crops will grow in a given area, they consult a farmer, a person who has technical, scientific knowledge of farming. They don't consult just anyone, nor do they trust the opinions of random people. They value the opinion of one who has scientific knowledge.

There are two ways in which Protagoras can stand against these arguments. Socrates himself points out these ways. First, it would seem that an individually centered Protagorean view does not allow for true knowledge and wisdom. However, if one understands the Protagorean argument in a social context, that is, that values and morals and truth are relative to societies, then there is still the potential for true knowledge and wisdom within a given society. Furthermore, it is possible to draw a distinction, as Socrates does, between the life of the true philosopher and the life of the Protagorean. A philosopher requires leisure and freedom and time for thought. One who maintains a Protagorean position, however, may engage in democratic politics. A philosopher must search for absolute truth. A sophist may search for immediate, relative policies to further the interests of their society.