SophistPlato

The Sophist is Plato's most technically demanding dialogue on the nature of being, not-being, and falsehood. An unnamed Visitor from Elea leads the conversation, attempting to define the sophist by the method of division, repeatedly cutting a genus into species until the target is isolated.

The central problem is that the sophist trades in appearances and falsehoods, but to say that falsehood exists is to say that what-is-not somehow is. Parmenides had declared that not-being cannot be spoken or thought. If he is right, falsehood is impossible, and the sophist is unassailable. Plato must therefore commit what he calls "parricide" against father Parmenides. He argues that not-being is not sheer nothingness but otherness: to say that something is not is to say that it is other than something else. The Form of the Other pervades all the Forms, making negation and difference possible without violating the intelligibility of being.

This move produces Plato's theory of the five greatest kinds: Being, Same, Other, Rest, and Motion. Each participates in some of the others (Being partakes of both Rest and Motion) and is other than each of the others. The Sophist thereby provides the metaphysical infrastructure for predication, negation, and the combination of Forms that the middle dialogues had left unanalyzed. It is the closest Plato comes to formal ontology.