The Meno is probably most famous for setting out the doctrine of anamnesis: the notion that your immortal soul possesses knowledge it has forgotten at your birth, but under the right conditions (i.e. if you are lucky enough to have Socrates ask you some strategic questions) you can be prompted to remember it. Socrates demonstrates this by leading an uneducated young slave through some geometry. Drawing figures in the sand, he gets the boy to “recollect” a special case of the Pythagorean theorem: that the square of the hypotenuse of an isosceles right triangle is double the square of one side (there is a nifty animated diagram of this on wikipedia).
Part of the process involves disabusing the boy of the notion that he knows the answer to begin with. Asked how to make a second square double the size of a first, the boy answers first that the sides should be twice as long – but when Socrates draws this square it become clear to the boy that it is four times as big, not double. The boy is perplexed; he thought he knew the answer, but has now realized that he didn’t. This is precisely the same experience Meno has just had – he thought he knew what virtue is, but questioned by Socrates it becomes clear that in fact he did not. In frustration, he quips to Socrates that “both in your appearance and in other respects you are extremely like the flat torpedo sea-fish; for it benumbs anyone who approaches and touches it” (80a).
UPDATE: At the Chicago Art Institute I found this fish plate (4th century Campanian) showing a torpedo fish! So now you can see what Plato’s audience would have pictured when Meno contends that Socrates looks like one as well as behaving like one:
The torpedo fish is the one on the left.
Discouraged to find that he is after all ignorant, Meno despairs of learning anything at all; like Donald Rumsfeld, he wonders how it is possible to know what you don’t know (“even supposing, at the best, that you hit upon it, how will you know it is the thing you did not know?” 80d). Anamnesis is Socrates’s answer to this paradox.
The tantalizing promise that the dialogue holds out is that human values (virtue, justice, temperance) are as pure and unambiguous and recognizable as geometrical figures. When someone draws for us the diagonal line in a square, we see that it divides the space exactly in half; Socrates implies that the certainty we feel about that could be replicated in certainty about virtuous or just behavior. If only someone would remind us how that looks.
After the nifty geometrical section Meno and Socrates go on, first to decide that virtue is teachable (it’s like knowledge) then that it is not (virtuous fathers fail to teach virtue to their sons). Finally Socrates suggests that virtuous people are virtuous through divine dispensation (evidently this is different than by “nature” although I’m not sure exactly why). Since nobody seems to have exact understanding of virtue (or else presumably there would be teachers of it) we are guided by “right opinion” rather than actual knowledge.
This is an interesting concept which Socrates illustrates with an example of somebody giving directions to a town. Presumably if he knows the correct way to get there, he can give us directions even if he has never been there himself; i.e. he can have the “right opinion” about how to get there even in the absence of true knowledge. When Meno understandably asks what the difference is between this and knowledge, Socrates gives a wonderful analogy of the moving statues of Daedalus; “if they are not fastened up they play truant and run away” (97d). In the same way, you might have right opinions, “but they do not care to stay for long, and run away out of the human soul, and thus are of no great value until one makes them fast with causal reasoning” (98a).
The first part of this process is the numbing, stunning, confusing realization that what we thought we knew we probably didn’t; our encounter with the torpedo-fish. But it is exactly when we become aware of our ignorance that we are obliged to forge ahead in search of what we now know that we didn’t understand:
“Some things I have said of which I am not altogether confident. But that we shall be better and braver and less helpless if we think that we ought to enquire, than we should have been if we indulged in the idle fancy that there was no knowing and no use in seeking to know what we do not know;—that is a theme upon which I am ready to fight, in word and deed, to the utmost of my power” (86b).