Socrates famously said that he didn't know anything—except the fact that he didn't know anything. But though he may not have had a positive conception of what constitutes knowledge, he certainly knew a bad argument when he heard one. In the Theaetetus, Socrates deploys his razor-sharp, questioning mind to dispose of a number of defective theories advanced by his eponymous interlocutor. Essentially, what Socrates is doing is clearing away the growth of false opinions to allow Plato to take the stage and present his own fully worked out epistemology, or theory of knowledge.
Socrates begins his dialog with Theaetetus by asking what seems like a fairly simple question: What is knowledge? The mathematician Theodorus is so overawed by the question and its implications that he absents himself from the subsequent debate, leaving Socrates and Theaetetus to get to grips with this knotty philosophical problem.
At first, Theaetetus tries to answer the question by giving specific examples of knowledge: the practical knowledge you get from a geometer, the theoretical knowledge you get from a mathematician, and so on. But Socrates isn't interested in specific types of knowledge; he wants a definition of knowledge itself, to get at the very essence of knowledge. There may well be many different kinds of knowledge, but Socrates wants to know what it is that they all share.
Undaunted by the failure of his first attempt at answering Socrates's question, Theaetetus—with the philosopher's encouragement—valiantly tries again. This time, he argues that knowledge is equated with sense perception. Socrates says that this is a good start and draws the young man's attention to similar theories put forward by Protagoras and Heraclitus.
But then Plato comes along and rains on Theaetetus's parade. He points out that perception is a weak foundation for knowledge and uses a number of examples to drive home his point. For instance, wine tastes sweet to Socrates when he's feeling well, but it is bitter when he's sick. How, then, can we base knowledge—sure and certain knowledge—on perception, something that's prone to such sudden and rapid change?
According to Plato, knowledge must be stable, fixed, and immutable. It cannot be based on differences—mere accidents—but must be based only on essences, things that are shared. The material world around us, the world of nature, is in a state of perpetual change. As such, it cannot provide the basis for the kind of knowledge we seek. So sense perception is ruled out as a source of knowledge—it can only provide opinion.
So Theaetetus offers another suggestion: knowledge is true judgment. But Socrates refutes the young man by using the example of a jury in a trial. It's certainly the case that a jury, convinced by a lawyer skilled in the arts of rhetoric, can be persuaded of the truth of his arguments, but that doesn't mean that they have knowledge. In other words, a jury can make a true judgment without being in possession of knowledge, simply because they've been swayed by compelling rhetoric.