Theory of Forms
Plato's doctrine that what is most real are eternal, unchanging Forms — Justice, Beauty, the Good — of which sensible things are imperfect images.
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.