In the Theaetetus, Socrates poses the question: “What is the nature of knowledge?”. Theaetetus' first answer is that the nature of knowledge is perception. If knowledge is perception, then each person has different knowledge. One person may feel that the wind is cold, while another person may perceive the very same wind as being warm. The Sophist Protagoras, summarizes this view: “Man is the measure of all things”.
If man is the measure of all things, then nothing is stable. One man might measure (that is, perceive) the wind as being cold, while another might measure it as warm. There is no stability in this measure, no external reference by which to claim what the wind really is. This means that the properties of the wind, and anything else that people might measure are in perpetual flux, a theory associated with the philosopher Heraclitus.
If everything is in flux, then it’s impossible for man to measure anything. A man is incapable of claiming that the wind is cold because the property of the wind is never stable; it changes depending on who you ask. If everything is in flux, nothing can be measured and thus there is no perception to be had because everything is chaos. If Theaetetus is basing his claim on Heraclitus’ doctrine, then he is incapable of maintaining that knowledge is perception without falling into a contradiction, for his claim ultimately implies knowledge is whatever anyone perceives it to be. It can be perception or it can be anything else. Socrates avoids this contradiction by asserting that a wise man, rather than every man, is the measure of all things.