Two different things are called form, by two philosophers who taught each other, and almost all the confusion around the word comes from running them together.

For Plato, a form is a perfect pattern existing apart from the world, which things resemble. For Aristotle, a form is the organizing principle inside a particular thing, which makes it the kind of thing it is.

One is a separate realm. The other is a structure in the chair. Keep them apart and the rest is manageable.

The problem Plato was solving

Look around and everything is changing. This horse ages, that just act is over, this beautiful face will not stay. If changing particulars are all there is, three things follow that Plato found intolerable.

Nothing is stable. Nothing is fully knowable, since what changes can only be opined about. And there is no fact of the matter about what a horse is, or what justice is, beyond the shifting instances.

His answer: what is most real is not the visible particulars but eternal unchanging patterns — the Forms. The Form of Horse is what makes any horse a horse. Particular horses come and go while participating in it. The visible world is the derivative, not the original.

Four commitments come with this. The Forms are real, not concepts in anyone's head — they would be what they are had nobody ever existed. They are eternal: the Form of Triangle was the same before any triangle was drawn. They are reached by reason, not the senses — you see triangles, you understand triangularity. And particulars participate in them, which is where the trouble starts.

The extended case for this is the three analogies in Republic VI–VII — the Sun, the Divided Line, and the Cave, which has its own entry.

Plato takes it apart himself

The strongest objection to the theory of Forms is in a dialogue by Plato, delivered by Parmenides to a young and outmatched Socrates. It is called the Third Man.

If particular men are men by participating in the Form of Man, then the Form and the particulars have something in common — that is what makes the participation work. But whatever they have in common is itself a Form, by the theory's own rule. Call it a second Form of Man.

Now the second Form and everything under it have something in common. That requires a third. The regress does not stop.

What makes this remarkable is not that the objection exists but who made it. Plato states the decisive attack on his own central doctrine and does not answer it in the dialogue. Scholars have argued for two and a half millennia about whether he thought it fatal, whether the regress genuinely follows, and what the late dialogues were trying to repair.

Aristotle's answer

Aristotle studied under Plato for twenty years and rejected the separation.

Form is real, he agreed. It is not somewhere else. The form of this horse is in this horse — its organization, what makes the matter a horse rather than a heap. There is no Horse over and above horses, and if every horse died the form would not persist in a separate realm; it would simply not be instantiated.

This is hylomorphism: every individual thing is a compound of matter — what it is made of — and form — what it is. The bronze and the shape; the statue is the composite. It is developed further under substance, which is where the consequences live.

The move looks technical and it is the hinge of Western metaphysics. It keeps the explanatory work Plato's Forms were doing — there is a real answer to what a thing is, and it is not just a name we use — while removing the separate realm and the regress that came with it.

The disagreement between teacher and student on this one question organizes most of the next two thousand years.

Where the Forms went

They did not die. Plotinus relocated them into the divine Intellect. Augustine carried that into Christian theology by identifying them with ideas in the mind of God — creatures imitate divine exemplars — and that became standard medieval doctrine, developed by Aquinas.

Notice what the move accomplishes. The Forms stop being free-standing eternal entities competing with God for ultimacy, which was always awkward for a creation theology, and become God's own eternal ideas. The metaphysics is preserved and the theology is protected.

The empiricists rejected the whole apparatus. Locke, Berkeley and Hume held that generality is a fact about words and habits of mind, not about the furniture of reality — the position now called nominalism.

It is still a live position

The strongest modern descendant is mathematical Platonism, and it is held by serious people for a serious reason.

Mathematical truths look discovered, not invented. Nobody decided that the primes are infinite; it was found, and it would have been true had no one looked. Mathematicians describe their work in the language of exploration rather than construction. And mathematics turns out to describe the physical world with an accuracy that is hard to explain if it is merely a convention we adopted.

If mathematical objects exist independently of minds, they are abstract, eternal, unchanging and known by reason rather than sense. That is Plato's list.

The standing objection is also Plato's problem: if these objects are outside space and time, how does a physical brain get into contact with them? Nobody has a satisfying answer, which is exactly where participation left off.

Why it is worth your time

The fight over Forms is the original version of a question you will meet in other clothes: when we group things together, are we finding a structure that is there or imposing one that is useful?

It is the same question as whether a diagnostic category carves something real, whether a market segment exists or was invented by the person selling to it, whether a species is a fact or a convenience.

Plato says the structure is there and is more real than the instances. Aristotle says it is there and only in the instances. The nominalists say we made it and it works. Most people hold all three, in different areas, without noticing.

Further reading