The most illustrious student Socrates had in philosophy was Plato, whose beautifully written dialogues not only offered an admiring account of the teachings of his master but also provided him with an opportunity to develop and express his own insightful philosophical views. In the remainder of our readings from Platonic dialogues, we will assume that the "Socrates" who speaks is merely a fictional character created by the author, attributing the philosophical doctrines to Plato himself. In the middle and late dialogues, Plato employed the conversational structure as a way of presenting dialectic, a pattern of argumentation that examines each issue from several sides, exploring the interplay of alternative ideas while subjecting all of them to evaluation by reason.

Plato was a more nearly systematic thinker than Socrates had been. He established his own school of philosophy, the Academy, during the fourth century, and he did not hesitate to offer a generation of young Athenians the positive results of his brilliant reasoning. Although he shared Socrates's interest in ethical and social philosophy, Plato was much more concerned to establish his views on matters of metaphysics and epistemology, trying to discover the ultimate constituents of reality and the grounds for our knowledge of them.

Plato's (Meno) is a transitional dialogue: although it is Socratic in tone, it introduces some of the epistemological and metaphysical themes that we will see developed more fully in the middle dialogues, which are clearly Plato's own. In a setting uncluttered by concern for Socrates's fate, it centers on the general problem of the origins of our moral knowledge.

The Greek notion of [aret], or virtue, is that of an ability or skill in some particular respect. The virtue of a baker is what enables the baker to produce good bread; the virtue of the gardener is what enables the gardener to grow nice flowers; etc. In this sense, virtues clearly differ from person to person and from goal to goal. But Socrates is interested in true virtue, which (like genuine health) should be the same for everyone. This broad concept of virtue may include such specific virtues as courage, wisdom, or moderation, but it should nevertheless be possible to offer a perfectly general description of virtue as a whole, the skill or ability to be fully human. But what is that?

When Meno suggests that virtue is simply the desire for good things, Socrates argues that this cannot be the case. Since different human beings are unequal in virtue, virtue must be something that varies among them, he argues, but desire for one believes to be good is perfectly universal Since no human being ever knowingly desires what is bad, differences in their conduct must be a consequence of differences in what they know. (Meno 77e) This is a remarkable claim. Socrates holds that knowing what is right automatically results in the desire to do it, even though this feature of our moral experience could be doubted. (Aristotle, for example, would later explicitly disagree with this view, carefully outlining the conditions under which weakness of will interferes with moral conduct.) In this context, however, the Socratic position effectively shifts the focus of the dialogue from morality to epistemology: the question really at stake is how we know what virtue is.

For questions of this sort, Socrates raises a serious dilemma: how can we ever learn what we do not know? Either we already know what we are looking for, in which case we don't need to look, or we don't know what we're looking for, in which case we wouldn't recognize it if we found it. (Meno 80e) The paradox of knowledge is that, in the most fundamental questions about our own nature and function, it seems impossible for us to learn anything. The only escape, Socrates proposed, is to acknowledge that we already know what we need to know. This is the doctrine of recollection, Plato's conviction that our most basic knowledge comes when we bring back to mind our acquaintance with eternal realities during a previous existence of the soul.

The example offered in this dialogue is discovery of an irrational number, the square root of 2. Socrates leads an uneducated boy through the sophisticated geometrical demonstration with careful questions, showing that the boy somehow already knows the correct answers on his own. All of us have had the experience (usually in mathematical contexts, Plato believed) of suddenly realizing the truth of something of which we had been unaware, and it does often feel as if we are not really discovering something entirely new but rather merely remembering something we already knew. Such experiences lend some plausibility to Plato's claim that recollection may be the source of our true opinions about the most fundamental features of reality. (Meno 85d) What is more, this doctrine provides an explanation of the effectiveness of Socratic method: the goal is not to convey new information but rather to elicit awareness of something that an individual already knows implicitly.

The further question of the dialogue is whether or not virtue can be taught. On the one hand, it seems that virtue must be a kind of wisdom, which we usually assume to be one of the acquirable benefits of education. On the other hand, if virtue could be taught, we should be able to identify both those who teach it and those who learn from them, which we cannot easily do in fact. (Meno 96c) (Here Socrates offers a scathing attack on the sophists, who had often claimed that they were effective teachers of virtue.) So it seems that virtue cannot be taught. Plato later came to disagree with his teacher on this point, arguing that genuine knowledge of virtue is attainable through application of appropriate educational methods.

Perhaps our best alternative, Socrates held, is to suppose that virtue is a (divinely bestowed?) true opinion that merely happens to lack the sort of rational justification which would earn it the status of certain knowledge. Whether or not we agree with this rather gloomy conclusion about the unteachability of virtue, the distinction between genuine knowledge and mere true opinion is of the greatest importance. For philosophical knowledge, it is not enough to accept beliefs that happen to be true; we must also have reasons that adequately support them.

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Plato: Immortality and the Forms