In Meno, Plato's character (and old teacher) Socrates is challenged by Meno with what has become known as the sophistic paradox, or the paradox of knowledge:
Meno And how are you going to search for [the nature of virtue] when you don't know at all what it is, Socrates? Which of all the things you don't know will you set up as target for your search? And even if you actually come across it, how will you know that it is that thing which you don't know?
In other words, if you don't know what the knowledge looks like, you won't recognise it when you see it, and if you do know what it looks like, then you don't need to look for it; either way, then, there's no point trying to gain knowledge.
Plato's response is to develop his theory of anamnesis. He suggests that the soul is immortal, being repeatedly incarnated; knowledge is actually in the soul from eternity (86b), but each time the soul is incarnated its knowledge is forgotten in the shock of birth. What we think of as learning, then is actually the bringing back of what we'd forgotten. (Once it has been brought back it is true belief, to be turned into genuine knowledge by understanding.) And thus Socrates (and Plato) sees himself, not as a teacher, but as a midwife, aiding with the birth of knowledge that was already there in the student.
In Phaedo, Plato develops his theory of anamnesis, in part by combining it with his theory of Forms. First, he tells us more about how anamnesis can be achieved; whereas in Meno we're given nothing but the method of questioning with which Socrates proceeds, in Phaedo Plato presents us with a way of living our lives so that we can overcome the misleading nature of the body through katharsis (Greek: καθαρσις; “cleansing” (from guilt or defilement), “purification”). The body and its senses are the source of error; knowledge can only be regained through the use of our reason, contemplating things with the soul (see 66 b–d).
Secondly, he makes clear that genuine knowledge, as opposed to mere true belief, is distinguished by its content. One can only know eternal truths, for they are the only truths that can have been in the sould from eternity. Though it can be very useful to have a true belief about, say, the best way to get from London to Oxford, such a belief can't count as knowledge; how could our souls have known for all eternity a fact about places that have existed for less than 2,000 years?
Sources and references
Plato Phaedo, 1911: edited with introduction and notes by Hohn Burnet (Oxford: Clarendon Press)
Jane M. Day 1994 Plato's Meno in Focus (London: Routledge) — contains an introduction and full translation by Day, together with papers on Meno by various philosophers
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