Theory of Forms

The theory of Forms is the metaphysical centerpiece of Plato's mature philosophy, developed in dialogues including the Republic, the Phaedo, and the Symposium. Plato holds that what makes any sensible thing the kind of thing it is — what makes a particular act just, or this circle a circle — is its participation in a Form: an eternal, unchanging, mind-independent entity that exists outside the world of becoming.

The Forms are the proper objects of philosophical knowledge, and the Form of the Good is the source of intelligibility for all the other Forms, just as the sun is the source of visibility for sensible things. Aristotle was Plato's most important student and most important critic on this point: he rejected the separate existence of the Forms and held that the universal exists in the particular, not apart from it. The dispute between Platonic realism and Aristotelian immanent realism has structured nearly every subsequent debate over universals.

Plato's mature dialogues develop the theory through several distinct lines of argument. The argument from opposites in the Phaedo holds that sensible things are F and not-F at once, so what F is itself must be something other than any sensible thing. The argument from recollection in the Meno claims that the slave-boy's geometrical knowledge presupposes acquaintance with what he could not have learned through experience. The Symposium presents the ascent toward the Form of Beauty as a progressive abstraction from particular beautiful things.

The dispute over the Forms structured medieval debates over universals. Realists from Boethius through Anselm and Aquinas held in different ways that universals are real, though most rejected Plato's separation. Nominalists from Roscelin through William of Ockham held that only individuals exist, with universals as mental signs. The dispute reappears in modern philosophy in debates over abstract objects, mathematical Platonism, and the metaphysics of properties.