The theory of Forms, theory of Ideas, Platonic idealism, or Platonic realism is a metaphysical theory, attributed to the Classical Greek philosopher Plato, suggesting that the physical world is not as real or true as timeless, absolute, unchangeable ideas. Ideas according to this theory, conventionally capitalized and translated as "Ideas" or "Forms", are the non-physical 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) of his dialogues who sometimes suggest that these Forms are the only objects of study that can provide knowledge. The theory itself is contested from within Plato's dialogues, and it is a general point of controversy in philosophy. Nonetheless, the theory is considered to be a classical solution to the problem of universals.
The early Greek concept of form precedes attested philosophical usage and is represented by a number of words mainly having to do with vision, sight, and appearance. Plato uses these aspects of sight and appearance from the early Greek concept of the form in his dialogues to explain the Forms and the Good.
The original meaning of the term εἶδος (eidos), "visible form", and related terms μορφή (morphē), "shape", and φαινόμενα (phainomena), "appearances", from φαίνω (phainō), "shine", Indo-European *bheh2- or *bhā- remained stable over the centuries until the beginning of Western philosophy, when they became equivocal, acquiring additional specialized philosophic meanings. Plato used the terms eidos and idea (ἰδέα) interchangeably.
The pre-Socratic philosophers, starting with Thales, noted that appearances change, and began to ask what the thing that changes "really" is. The answer was substance, which stands under the changes and is the actually existing thing being seen. The status of appearances now came into question. What is the form really and how is that related to substance?
The Forms are expounded upon in Plato's dialogues and general speech, in that every object or quality in reality—dogs, human beings, mountains, colors, courage, love, and goodness—has a form.