There is not a great deal of context that is crucial to understanding the essential themes of the Meno, largely because the dialogue sits nearly at the beginning of western philosophy. Socrates and Plato are working not so much in the context of previous philosophies as in the context of the lack of them. Further, this is very probably one of Plato's earliest surviving dialogues, set in about 402 BCE (by extension, we might presume that it represents Socrates at a relatively early stage in his own thought). Nonetheless, in order to understand the aims and achievements of the dialogue, it helps to keep in mind some details about this lack of previous philosophies.
Since neither virtue nor any other concept has yet been defined in the way to which we are now accustomed, Socrates has to show that defining these things at all is a good idea. In this task, his primary foe is Greek cultural custom and the political aristocracy that most strongly embodies that custom. Meno, a prominent Thessalian who is visiting Athens, is a member of this class. Meno's semi-foreign status aids Socrates (and Plato) in the dialogue, allowing for eyewitness accounts that Socrates himself could not give. Thus, Meno is able to say with authority that the Thessalians do not have anyone who can clearly teach virtue, while Socrates (and Anytus, a prominent Athenian statesman) can vouch for the sorry state of affairs in Athens.
Meno is also a handy interlocutor for this dialogue because he is a follower of Gorgias, one of the most reputable of the Sophist teachers, and knows the Thessalian Sophist community to some extent. He therefore serves as a Sophist foil for Socrates' logical points. This is not quite a fair fight, of course, since Plato can put whatever words he wants in Meno's mouth, and because Meno is not himself an accomplished Sophist (like Gorgias, who is the central figure in a much lengthier Platonic dialogue).
Nonetheless, Socrates sets Meno up early on as a naive believer in the kind of pompous, elaborately rhetorical, but largely vacuous Sophist method of philosophy that had come to prominence some forty or fifty years earlier. Meno readily admits to being an enthusiastic follower of Gorgias and implicitly agrees to Socrates' characterization of Sophist arguments as bold, grand, and presumptuous. In this sense, Meno is something of a straw man set up by Plato to highlight the kind of philosophy Socrates wants to denounce. Meno clearly prefers the Sophist-style definition of color offered by Socrates to the plain, direct definition of shape that Socrates himself prefers.
If Meno is something of a dummy for aristocratic Sophist sympathizers, Anytus is even more clearly a stand-in for the somber, unconsidered values of the Athenian political elite. An actual historical politician of the time, he's grumpy, largely closed to new ideas, and insistent on inherited, class-based customs as the vehicle for virtue--he suggests that any "gentleman" on the streets of Athens is a fine example of virtue. Anytus, an Athenian conservative, despises the Sophists. Like other prominent Athenians at the time, he is probably suspicious of the Sophists' cleverness with words and their tendency to lead young followers away from success in worldly matters. Socrates encounters this idea that philosophy is a corrupting influence in many of his dialogues, and that perception will eventually lead to his trial and execution for "corrupting the youth." Thus, Plato is all the more determined to highlight Socrates' profound differences from the Sophists.
We should note briefly the basic form of the Platonic dialogues: Plato, Socrates' student, has written a kind of play, re-enacting the way in which Socrates practiced his philosophy (he did not write it down, but simply argued on the streets). In reading the summary contained on this site, it may not always be clear that Socrates is constantly asking questions of Meno, and only rarely offering points himself. The basic form of this kind of Socratic interview (the elenchus) is for Socrates to get his interviewee to admit to Socrates' points in response to questions (although, in practical terms, these answers are often just affirmations of what Socrates puts forth). This method is deeply related to the idea of anamnesis--the interlocutors are thought to be actually "recalling" the truths set out in the dialogues, rather than simply learning them from Socrates. The examination of Meno's slave is an excellent microcosm of this process.
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