The Third Man Argument

Plato's theory of Forms says particular large things are large by sharing in one perfect Form of Largeness. But in the Parmenides, Plato turns this against himself: if that Form is itself large, then it and the ordinary large things have largeness in common, which seems to require yet a further Form to explain the resemblance—and so on without end. Aristotle later sharpened the objection and gave it its enduring nickname, the "Third Man." It remains one of the most penetrating self-criticisms in the history of philosophy, exposing the cost of treating a Form as both a standard and an example of the quality it defines.