Plato’s Theory of Forms Explained

Plato’s Theory of Forms (sometimes called the Theory of Ideas) is one of the most famous and most influential philosophical theories in the history of Western thought. First developed in Plato’s middle-period dialogues — Phaedo, Republic, Symposium, Phaedrus — the Theory claims that the changing, imperfect objects of the everyday world are imperfect copies of eternal, perfect, unchanging Forms (or Ideas) that exist in a separate realm. The Theory is the foundation of Plato’s philosophy, the source of his famous metaphors (the Divided Line, the Cave, the Winged Chariot), and the inspiration for centuries of metaphysical and theological thought.

This page is a concise introduction to Plato’s Theory of Forms. It explains what the Forms are, what the theory claims, what the arguments for it are, and what its legacy has been. It links back to the Greek Philosophy cluster, the Who Was Socrates? page, and the Aristotle’s Contributions page.

The Problem

Plato was troubled by the appearance of change, imperfection, and disagreement in the world around him. The horse in front of you is beautiful, but it is not perfectly beautiful; a different horse, even more beautiful, will refute your claim that the first horse is beautiful. The just person you are talking to is just, but in a different sense from the just law or the just institution. The bed you are sitting on is a bed, but in a different sense from the bed in the carpenter’s mind or the bed in the bedmaker’s craft. How can we have stable knowledge of anything, if the world is constantly changing and the same word can refer to many different things?

The pre-Socratic philosophers had tried to find a single underlying substance (water, air, fire, the boundless) to explain the world, but Plato was unconvinced. He wanted a theory that could explain both the stability of knowledge and the variability of the world.

The Theory

Plato’s answer was the Theory of Forms. According to the theory, there exists a separate realm of eternal, perfect, unchanging Forms (sometimes translated as “Ideas”) that are the originals of which the objects of the everyday world are imperfect copies. The Form of Beauty is the perfect, eternal Beauty of which all beautiful things in the world are imperfect copies. The Form of Justice is the perfect, eternal Justice of which all just acts, just persons, and just institutions are imperfect copies. The Form of a Horse is the perfect, eternal Horse of which all horses in the world are imperfect copies.

The Forms are not in the everyday world, and they are not in space and time. They exist in a separate realm, accessible only to the mind. The philosopher’s task is to recognize the imperfection of the everyday world, to recognize that the everyday world is a copy of the Forms, and to use reason to ascend from the copies to the originals.

The Famous Metaphors

Plato used several famous metaphors to explain the Theory. The most important are:

The Divided Line (in the Republic). Plato divides a line into four segments. The lowest segment represents images (shadows, reflections, pictures). The next segment represents the things that are the originals of the images (the objects in the everyday world). The third segment represents the mathematical sciences, which study the abstract forms of the everyday world. The highest segment represents the Forms themselves.
The Cave (in the Republic). Plato imagines a cave in which prisoners are chained from birth, able only to see the shadows cast on the wall by a fire behind them. The shadows are the everyday world; the prisoners’ interpretation of the shadows is the popular view of reality. One prisoner escapes, sees the fire and the world outside, and returns to tell the others. The others do not believe him and may even kill him. The escaped prisoner is the philosopher; the cave is the everyday world; the world outside the cave is the realm of the Forms.
The Winged Chariot (in the Phaedrus). Plato compares the soul to a chariot drawn by two winged horses — one noble, one ignoble — and driven by a charioteer. The charioteer represents reason; the noble horse represents the spirited, courageous part of the soul; the ignoble horse represents the appetites. The chariot rises to the realm of the Forms, where the charioteer can see the Forms of Justice, Temperance, Knowledge, and Beauty. The Theory of Forms is, in this metaphor, the vision of the charioteer.

The Arguments for the Theory

Plato offers several arguments for the Theory of Forms. The most important are:

The Argument from the Sciences. The sciences (mathematics, geometry, astronomy) deal with abstract objects (triangles, numbers, circles) that are not the same as the particular triangles, numbers, and circles of the everyday world. The science of geometry deals with the perfect triangle, not the imperfect triangles drawn in the sand. The Forms are the objects of the sciences.
The Argument from the Parmenides. If a horse and a beautiful horse are both horses, there must be a Form of Horseness (and a Form of Beauty) in which they both participate. This argument is developed in Plato’s dialogue Parmenides, which also raises some of the most serious objections to the theory.
The Argument from Recollection. In Plato’s dialogue Phaedo, Socrates argues that the soul has knowledge of the Forms before birth, and that learning is a kind of recollection of this pre-natal knowledge.

The Criticisms of the Theory

Plato’s most famous critic was his own student, Aristotle. In his Metaphysics, Aristotle argued that the Theory of Forms is useless as an explanation of the world: the Forms are just “another set of beings” that do the same work as the everyday objects they are supposed to explain. Aristotle’s own philosophy rejected the Theory of Forms in favor of a more empirical approach to the world.

Plato’s own later dialogues (Sophist, Parmenides, Philebus) raise further objections to the theory. The so-called “third man argument” — if a horse and a beautiful horse are both horses, there must be a Form of Horseness in which they both participate; but then the original horse and the Form of Horseness are both horses, and there must be a second Form of Horseness in which they participate — seemed to show that the theory leads to an infinite regress.

The Legacy of the Theory

The Theory of Forms has had an enormous influence on Western philosophy. The Neoplatonists (Plotinus, Porphyry, Proclus) reinterpreted the Forms as the thoughts of God, and this Neoplatonic reading was taken up by the Christian theologians Augustine, Boethius, and the medieval Scholastics. The Theory also shaped the development of the concept of the universals in medieval philosophy (the debate over whether universals exist independently of particular things or only in the mind), and it was eventually combined with Christian theology in the work of Thomas Aquinas and others.

In the modern period, the Theory of Forms has been both celebrated and criticized. The Cambridge Platonists of the seventeenth century, the German Idealists (Hegel, Schopenhauer), and the British Idealists (Bradley, McTaggart) all built philosophical systems that drew heavily on Plato. The empiricists (Locke, Berkeley, Hume) and the analytic philosophers (Russell, Moore, Wittgenstein) generally rejected the Theory, but the questions it raised continue to be debated in contemporary philosophy of mind, metaphysics, and ethics.