Plato returns to his criticism of Protagoras’s claim that man is the measure of all things. Granted that such an argument is wrong (and silly), we explore the nature of knowledge and why it can’t be sense impression.
Theaetetus has just come back from the Sophists who argue that knowledge = sense perception. The larger context is Protagoras’s claim that “man is the measure of all things.” We will call this claim (P). We will distinguish this from Theaetetus’s claim that knowledge is perception, called (T).
Socrates asks him that if (T) is true, then knowledge must also be perceiving, to which Theaetetus agrees. If this is true, then a thing’s appearing-to-me must also be a thing’s being or existence. Our claim now entails that such knowledge is unerring (since it is connected with being). This, however, is manifestly false. Case in point: we perceive things in dreams, but no one thinks dreams are real.
Theaetetus retreats from this claim and attacks from the Heraclitean point of view that “motion is the source of being.” Flux, not stability is primary. There is no self-existent thing. Everything is becoming and in relation. He has the nice phrase “Partisans of the perpetual flux.” Indeed, we can’t even say man or stone, but only an aggregate of x. This is word-for-word Karl Marx (Marx, Theses on Feuerbach, Thesis VI).
Let’s return to (P). If it is true, then there is no reason to believe that Protagoras (or the modern university professor) is correct. If knowledge is sensation, and I can’t discern another man’s sensation, and yet Protagoras purports to be true, then why prefer him to anyone else? This was the first response to postmodernism long before postmodernism came on the scene.
Another problem: I can have knowledge from memory, yet memory isn’t a sense.
Another problem: I can have knowledge of abstract entities and categories, yet these aren’t present to the senses.
Let’s return to the Heraclitean claim. If nothing is at rest, and everything is supervening upon everything else, then every answer is equally right, since all we have are moving targets.
There is yet another diversion where Socrates explains that the soul perceives some things by herself and others by means of bodily organs. The soul has something like “wax” in it that handles the impressions. If a soul is deep and virtuous, then the impressions sink to the heart of the soul.
The dialogue ends with discussions of justified, true belief.
Arguably the most important of his “epistemology” dialogues, it is somewhat a difficult read as Socrates goes through numerous diversions.