The Theory of Forms

Plato's most famous idea is the theory of Forms. He noticed that we recognize many things as beautiful, equal or just, yet every example in the physical world is imperfect and changing: a beautiful face ages, and two sticks are never exactly equal. Plato concluded that there must be perfect, unchanging realities, the Forms, such as Beauty itself or Equality itself, which physical things only imitate or 'participate in'.

The Forms are grasped by reason rather than the senses, and true knowledge is knowledge of them, while beliefs about the changing world are mere opinion. The theory combined Heraclitus' insight that the physical world constantly changes with Parmenides' demand for something stable to know. It also raised problems that Plato himself explored in the Parmenides, such as how a single Form can be present in many things. Aristotle later rejected separate Forms, starting a long debate.