Plato: Knowledge as justified true belief
Plato is the first Western philosopher who left us a substantial body of writing. But he didn't write philosophical treatises like Descartes' Discourse on Method.
Instead he wrote dialogues: dramatic works in which different characters represent and argue for different philosophical positions. (He did this more explicitly than Wittgenstein, who doesn't actually give names and personalities to the exponents of the different positions that are canvassed in the Philosophical Investigations.) In Plato's dialogues the central character is usually his teacher, Socrates, whose philosophical technique was to proceed not by stating a position but by asking questions and leading those with whom he talked to their own answers. (This is sometimes called the Socratic method.) In the dialogue called the Theaetetus, Socrates discusses the question “What is knowledge?” with a young man called Theaetetus. Because Plato's discussion of knowledge has been as central to the Western tradition as Descartes' view of mind has been to modern philosophical psychology, I want to begin considering what knowledge is by examining some of the ideas discussed by Socrates and Theaetetus in this famous dialogue.Theaetetus begins answering Socrates' question “What is knowledge?” by giving examples of knowledge: geometry, for example, and the technical know-how of a shoemaker. But Socrates objects that what he wants is not a bunch of examples of knowledge, but rather an explanation of the nature of knowledge. In answer to the philosophical question “What is knowledge?” what is wanted is a definition that we can use to decide whether any particular case really is a case where somebody knows something.
Theaetetus then makes other attempts at answering the question that do give definitions of this sort.
But Socrates argues against all of them. Finally, Theaetetus suggests that to know something is just to believe something that is true. If you know that you are reading this book, for example, then, on Theaetetuss theory,a) you must believe you are reading this book, and
b) you must, in fact, be reading this book.
Socrates points out that it follows from this theory of Theaetetus' that when a skilled lawyer persuades a jury that someone is innocent, then if the person is in fact innocent, the jury knows he or she is innocent, even if the lawyer has persuaded the jury by dishonest means. This consequence, Socrates argues, shows that Theaetetus' theory must be wrong, because in such circumstances we would not allow that the jurors knew that the accused person was innocent, even if they correctly believed it.
Socrates has a point. Suppose, for example, my lawyers believe that I am innocent and that I am being framed. They might decide that it was more important to protect someone from being framed than to respect the law, which the prosecutors are, after all, abusing. So they might fake “evidence” that undermines the fake “evidence” produced by the prosecutors. Suppose they persuaded the jury: the members of the jury would correctly believe I am innocent, but they certainly wouldn't know that I am innocent.
Here is the passage where Socrates summarizes his objection and Theaetetus responds:
SOCRATES: But if true belief and knowledge were the same thing, then the jury would never make correct judgments without knowledge; and, as things are, it seems that the two [knowledge and true belief] are different.
THEAETETUS: Yes, Socrates, there's something I once heard someone saying, which I'd forgotten, but it's coming back to me now. He said that true belief with a justification is knowledge, and the kind without a justification falls outside the sphere of knowledge.
Theaetetus realizes that this case shows that we need some third condition for knowledge: knowing does involve believing, and it does involve the truth of what you believe, but it also requires something else.
And, since he is nothing if not persistent, Theaetetus suggests that knowledge is true belief along with a justification. The rest of the Theaetetus is taken up with discussing what sort of justification is necessary. But the essential idea is that to know something,a) you must believe it,
b) it must be true, and
c) you must be justified in believing it.
It is the recognition that we need this third condition—which I'll call the justification condition—that is the Theaetetus’ major legacy to epistemology. That the justification condition and the first two conditions, taken together, are necessary and sufficient conditions for knowledge is a central philosophical claim of the Western tradition since Plato. This idea is often expressed in the slogan “Knowledge is justified true belief.”
Socrates never accepts any of Theaetetus' attempts to define exactly which kind of justification is necessary to turn true belief into knowledge, but the idea provides the starting point for many philosophical attempts to define knowledge since. Typically, philosophers have first argued for the view that knowledge is justified true belief and then gone on to ask the question “What kind of justification do you need in order to have knowledge?”
Theaetetus' idea is suggested by a diagnosis of why the jurors don't really know I'm innocent. That diagnosis is, roughly, that though the jurors have a true belief, it isn't one that they are entitled to have, since my lawyers could have used the very same evidence to convince them I was innocent, even if I had been guilty. In other words, the evidence my lawyers gave the jury for the claim that I was innocent was consistent with my being guilty, even though it persuaded them that I was not. This diagnosis is at the root of the first of two major ways in which philosophers have tried to say exactly what the justification condition amounts to. That account is found in the epistemology of Descartes.
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