structural:summarization, structural:expansion, information:filtering
In philosophy and specifically metaphysics, the theory of Forms, theory of Ideas, Platonic idealism, or Platonic realism is a theory widely credited to the Classical Greek philosopher Plato. The theory suggests that the physical world is not as real or true as "Forms". According to this theory, Forms—conventionally capitalized and also commonly translated as "Ideas" —are the non-physical, timeless, absolute, and unchangeable essences of all things, of which objects and matter in the physical world are merely imitations. Plato speaks of these entities only through the characters (primarily Socrates) in his dialogues who sometimes suggest that these Forms are the only objects of study that can provide knowledge.

Scriptures written by Pythagoras suggest that he developed a similar theory earlier than Plato, with Pythagoras's theory specifically proposing that the world is entirely composed of numbers. The early Greek concept of form precedes attested philosophical usage, and is represented by a number of words which mainly relate to vision, sight, and appearance. Plato uses these aspects of sight and appearance from the early Greek concept in his dialogues to explain his Forms, including the Form of the Good. The theory itself is contested by characters within Plato's dialogues, and it remains a general point of controversy in philosophy. Nonetheless, it is considered to be a classical solution to the problem of universals.