ParmenidesPlato
About Parmenides
The is Plato's most difficult dialogue and possibly his most consequential for the history of metaphysics. In it, a young Socrates presents the theory of Forms to the aged Parmenides, who subjects it to a series of devastating objections. Then Parmenides demonstrates his own method through an elaborate gymnastic exercise on the hypothesis "if the One is."
The objections in the first half are severe. If each Form is one, how can many things participate in it without the Form being divided? If we posit a Form to explain the likeness between things, we need another Form to explain the likeness between the first Form and the things (the "Third Man" regress). If the Forms are separate from the things that participate in them, how can we know them at all? Parmenides does not solve these problems. He tells Socrates that the theory needs more training, not abandonment.
The second half is the exercise: eight (or nine) deductions exploring what follows if the One is and if the One is not. The results appear contradictory. The One is both limited and unlimited, in motion and at rest, the same as and different from itself. Whether Plato intends these as genuine antinomies, as a reductio, or as a map of the necessary structure of any first principle has been debated since antiquity. Plotinus, Proclus, and Hegel all built from this dialogue, each reading the deductions as revealing something positive about the nature of unity and being.