There is an empiricist commonplace that identifies “knowledge” with “justified true belief”. This makes knowledge an especially good kind of belief. I regard that as a flat-out category mistake.
I want to suggest that knowledge is not a kind of belief or opinion at all. As usual, I don’t claim to “know” what I “suggest” with some force as interesting or worthy of consideration, so in particular I do not claim to know that knowledge is not belief or opinion. (I am also not trying to say exactly what knowledge is, only to delimit it somewhat.)
What I am doing is recommending a different use of the word “knowledge”, that at minimum distinguishes it from belief or opinion. This is based on the belief or opinion that the belief or opinion that “knowledge is not belief or opinion” is a well-founded belief or opinion.
I read Plato as very sharply distinguishing “knowledge” properly so-called (epistémé) from any kind of doxa (opinion or belief). This would rule out the identification of knowledge with justified true belief.
It is not uncommon, however, to see claims that Plato himself identified knowledge with justified true belief. I will offer a different interpretation of the main relevant passage here. I apologize for using the old Jowett translation, which is easily accessible online.
“Then when the jurors are rightly persuaded of something one could not actually know except by being present — when they judge it, that is, on hearsay, and yet with a true opinion, they judge it without knowledge; even though, if their decision is sound, their persuasion is correct” (Plato, Theaetetus 201).
Ignoring the particular criterion of knowledge mentioned in the example, the essential is that on reflection, we should all be able to agree that there are cases in which we would say that someone has a true opinion without knowledge. So far, this is agnostic to whether or not knowledge is justified true opinion. It just establishes that true opinion in itself is not knowledge.
“When therefore anyone conceives a true opinion of anything without a reasoned statement, his mind is free from error about it, but does not know it; for the man who cannot give and accept a reasoned statement about anything, has not knowledge of it: but when he adds to his true opinion a reasoned statement, he has in addition all that is required to constitute knowledge” (202).
Here it is very important to distinguish between statements about knowledge and statements about someone who has it. What is argued in the above quotation is that the person who has a true opinion and a reasoned statement has what she needs for knowledge.
I would agree that a person who has knowledge can reasonably be said also to believe what she knows. But it does not follow from this that knowledge itself is any kind of belief, or in particular that knowledge is just true belief accompanied by a reasoned statement. Nothing in the argument excludes the possibility that knowledge itself — as distinct from the person who has both knowledge and belief — is tied only to the reasoned statement, and is in itself independent of the person’s belief.
I think this is already sufficient to disprove the claim that this section of the Theaetetus expresses the view that knowledge is reducible to justified true belief.
If knowledge were tied only to the reasoned statement, it would still be true that the person who also had a true belief would have what she needed for knowledge. Again, I don’t mean to say that “reasoned statement” is sufficient by itself to define knowledge, even though I think it gives an important hint. It is worth noting, however, that Plato’s mention of a reasoned statement is more specific than the simple mention of justification.
Also, “truth” is said in more than one way. The kind of truth that could reasonably be said of a belief or opinion is only a correspondence to facts. The kind of truth of principal interest to Plato was very different from this.
I also think there is a broad category of acquaintance that is extremely important to humans, but is different from knowledge. The kind of experience I find interesting is mainly not ephemeral immediate experience, but the more substantial thing that we mean when we say someone is “experienced”. (See also Imagination, Emotion, Opinion; Consciousness, Personhood; A Criterion for Knowledge?; Everyday Belief; Belief is Different from Faith.)