Parmenides

Plato's most puzzling dialogue, in which his own theory of forms comes under attack. It describes a meeting at Athens between a very young Socrates and the visiting Eleatic philosophers Parmenides and Zeno. Socrates proposes forms, such as likeness and unlikeness, to answer Zeno's arguments against plurality. Parmenides then presses a series of objections: are there forms of hair, mud and dirt? Does a thing share in the whole form or only a part of it? If large things share in a form of largeness, does that create a new large thing needing another form, without end (the argument later called the Third Man)? And if forms are separate from us, how can we know them? Parmenides still says thought needs forms, and urges training. The second part is that training: a long, dense exercise on the hypotheses that the One is and that it is not, reaching conflicting conclusions.

Here Plato criticises himself more sharply than any later opponent, and the Third Man argument is still a test case for any theory of universals.