Platonic Idealism: The Platonic Ideal and Plato's Theory of Forms

Platonic idealism is the name usually given to Plato’s claim that the most real things are not the objects we see and touch but the Forms (in Greek eidos or idea): single, unchanging patterns such as Beauty itself, Justice itself and Equality itself. A face is beautiful, and a law is just, only because it shares in its Form. The face will age and the law will be repealed. The Form does neither.

That is Plato’s version. Most people meet the idea first in a looser, everyday form, when someone calls a perfect croissant “the Platonic ideal of a croissant”. The two are related, but they are not the same thing, and the difference is worth getting right. Below: what the phrase means, what Plato actually argued and where, why “idealism” is a misleading label for it, and the objections that Plato and his best student raised against it.

What does “the Platonic ideal” mean?

In ordinary English, the Platonic ideal of X is the perfect version of X: the model that every real example is measured against and falls short of. “The Platonic ideal of a diner breakfast”, “the Platonic ideal of a detective story”. The phrase suggests a single standard behind many imperfect cases, and it suggests that we recognise the imperfect cases by comparing them with that standard.

Those two features come straight from Plato. For him there are many beautiful things and one Beauty, many equal things and one Equality, and we judge the many by looking to the one. So the everyday phrase has kept the shape of the idea.

It has lost three things along the way.

• The everyday ideal lives in someone’s head. Your Platonic ideal of a croissant is a picture you carry around, and mine may differ. Plato’s Forms are not pictures in anyone’s mind. They exist whether or not anyone thinks of them.
• The everyday ideal is a very good example. The ideal croissant is still a croissant, just a flawless one. Plato’s Form of Beauty is not an extra-beautiful object sitting at the top of a league table. It is what beauty is, the thing that makes every beautiful object beautiful.
• The everyday ideal is a matter of taste. Plato’s Forms are not. They are the most objective things there are, and knowing them is the goal of philosophy.

So when you use the phrase, you are borrowing Plato’s vocabulary for a thinner idea. That is fine in conversation. In an essay on Plato it is worth saying which one you mean.

What is Platonic idealism, in plain terms?

Plato’s view, usually called the theory of Forms, rests on three connected arguments. (For a fuller account and a critical assessment, see our review of Plato’s theory of Forms.)

The senses cannot explain what we know. We see sticks and stones that are roughly equal, but never two things that are perfectly equal. Yet we have the idea of perfect equality, and we use it to notice that the sticks fall short. That standard did not come from the senses, because the senses never show it to us.

What keeps changing cannot be known. The physical world is always in flux, so about it we can only have opinion (doxa). Knowledge (episteme) needs an object that stays put. In the Timaeus (27d–28a) Plato draws the line between “what always is and never comes to be” and “what always comes to be and never is”. If knowledge exists at all, there must be something of the first kind for it to be about.

The many need a one. A thousand just acts differ in every way except in being just. What they share is not any one of them. It is Justice itself, one Form over many cases. As Socrates puts it in the Republic, “we usually posit a single Form for each group of many things to which we give the same name” (596a).

Put together, these give Plato’s picture of reality in two tiers. Below, the visible world of things that come and go. Above, the intelligible world of Forms, grasped by thought and not by sight, and more real than what copies them.

Where does Plato argue for the Forms?

The Forms turn up across the middle dialogues. Four passages do most of the work.

The equal sticks (Phaedo 74a–c)

In the Phaedo, on the day of his death, Socrates asks whether equal sticks and stones do not sometimes look equal to one person and unequal to another, while staying the same. They do. But “the equals themselves”, he says, never appeared unequal, nor Equality inequality (74c). So Equality itself is something other than the equal sticks. We must have known it before we first saw any sticks, since we use it to judge them. This is the link between the Forms and Plato’s theory of recollection, also set out in the Meno (81d): the soul knew the Forms before birth, and learning is remembering.

Beauty itself (Symposium 211a–b)

In the Symposium the priestess Diotima describes a ladder of love. It climbs from one beautiful body to all beautiful bodies, then to beautiful souls, laws and kinds of knowledge, and finally to Beauty itself. This Beauty, she says, “always is, and neither comes to be nor passes away, neither grows nor shrinks” (211a). It is not beautiful in one respect and ugly in another, or beautiful to some people and not to others. Everything else that is beautiful shares in it, and comes and goes while it stays the same. More on this in what beauty means for Plato.

The bed and the painter (Republic 596a–598b)

In Book 10 of the Republic, Socrates uses a humble example. There are many beds, but one Form of Bed. The carpenter looks to that Form when he makes a bed, so he does not make “what is” but “something like what is” (597a). Then the painter comes along and paints the carpenter’s bed, from one angle, as it appears. That gives three beds: the Form, the carpenter’s copy, and the painter’s copy of a copy (597b). The passage is mainly an attack on imitative art, but it is also the clearest picture Plato gives of the ladder of reality.

The Sun, the Line and the Cave (Republic 507b–517c)

The central books of the Republic give three linked images. In the analogy of the sun, the Form of the Good does for the Forms what the sun does for visible things: it makes them knowable and gives them their being (509b). The divided line (509d–511e) ranks four levels, from shadows and reflections, through physical things and mathematical objects, up to the Forms, and pairs each with a state of mind, from guesswork to understanding. The allegory of the cave (514a) turns the same scheme into a story: prisoners who take shadows for reality, and one who is dragged up into the sunlight. At the top of all three stands the Form of the Good, which Socrates calls “the greatest thing to learn” (505a).

Is Platonic idealism the same as Berkeley’s idealism?

No, and this is the most common confusion about the topic.

Plato never called himself an idealist. The word is modern: it came into use as a philosophical label around the start of the eighteenth century, and Leibniz was one of the first to apply it to Plato. It stuck because Plato’s word for a Form, idea, looks like our word “idea”. But in Greek idea originally means something like “look” or “shape”, the visible character of a thing, not a thought.

In modern philosophy “idealism” usually means that reality is in some way mental. George Berkeley (1710) held that to be is to be perceived: there is no mind-independent matter, only minds and their ideas. Kant’s transcendental idealism holds that the world as we experience it is partly shaped by the structure of our own minds.

Plato’s view is close to the opposite. The Forms are not ideas in anyone’s mind, human or divine. They exist independently, and thought discovers them the way an eye discovers what is lit. Plato even tests the mental reading and rejects it. In the Parmenides (132b–c), the young Socrates suggests that each Form might be a thought existing only in souls. Parmenides replies that a thought must be a thought of something that is, so the Form would be that something, not the thought. For this reason many philosophers call Plato’s position realism about universals, not idealism.

(A later twist: Christian Platonists such as Augustine placed the Forms in the mind of God, as the divine ideas by which he created. That is a development of Plato, not Plato’s own view.)

	Everyday “Platonic ideal”	Plato’s Form	Berkeley’s idealism
What it is	The best imaginable version of something	The one unchanging reality a kind of thing shares in	The view that only minds and their ideas exist
Where it exists	In the speaker’s imagination	Independently of any mind	In minds (finite minds and God’s)
Is it mental?	Yes	No	Yes, everything real is
Relation to physical things	A standard to compare them with	Their model and cause; they are its copies	Physical things are collections of ideas
How we know it	By taste or preference	By reasoning and dialectic, not the senses	By perceiving
Is it one more thing of its kind?	Yes, a perfect croissant is a croissant	No, Beauty itself is not one more beautiful object	Not applicable

What are the main characteristics of Platonic idealism?

Summed up, these are the features that define Plato’s view and set it apart from later idealisms.

1. Two orders of reality. There is the sensible world, which changes and is perceived, and the intelligible world of Forms, which does not change and is thought. Only the second is fully real.
2. The Forms are objective, not mental. Unlike the ideas of Berkeley or Kant, the Forms depend on no mind. They exist whether or not anyone thinks of them.
3. Participation. Physical things are what they are by sharing in, or imitating, a Form. A beautiful thing is beautiful by Beauty.
4. Hierarchy. The Forms are not a flat list. At the top is the Form of the Good, which gives the others their being and makes them knowable (Republic 509b).
5. Reason over the senses. The senses give opinion. Only thought gives knowledge. The cave is this claim told as a story.
6. Immortality and recollection. The soul knew the Forms before birth, and learning is recollecting them (Meno 81d; Phaedo 72e–77a).
7. Ethics and politics follow. If there is an objective Good, there is a good life and a just city that can be known, and ruling belongs to those who know them.

What are the main objections to Platonic idealism?

Plato’s own sharpest critic was Plato. In the first part of the Parmenides, an elderly Parmenides cross-examines a young Socrates and leaves the theory badly bruised.

Which things have Forms? Socrates is happy to posit Forms of Justice, Beauty and Good, but hesitates over hair, mud and dirt (130c–d). If the Forms are the patterns of things, why not these too? And if they do have Forms, the intelligible world starts to look less noble than advertised.

The Third Man. Take several large things. We see one Form, Largeness, over them. But now consider the large things together with Largeness itself: if Largeness is also large, then another Form of Largeness must appear over that whole group, and another over that, without end (132a–b). Each Form turns out to be not one but infinitely many. Aristotle later gave the argument the name it is known by, “the Third Man”, using the example of a man. The problem arises from treating a Form as a perfect instance of itself, which is exactly the mistake the everyday phrase “Platonic ideal” invites.

Aristotle’s critique. Plato’s student Aristotle rejected the separate world of Forms altogether. For him, forms exist in things: the form of this horse is in this horse, not in a heaven of its own. Positing separate Forms doubles the number of things to be explained without explaining anything, and to say that things “participate” in Forms is, in his words, to use “empty words and poetic metaphors” (Metaphysics I.9). Much of the later history of metaphysics can be told as an argument between these two positions. See Aristotle’s central critique of Plato for the detail.

Plato never published a clean answer to these objections. That he published them at all, in their strongest form, is one of the more admirable facts in the history of philosophy.

Why is mathematics the best modern case for Platonism?

If any part of Plato’s view still has wide support, it is the part about numbers and shapes. Plato was deeply influenced by the Pythagoreans, and his divided line puts mathematical objects just below the Forms, above everything physical.

The case is easy to feel. No drawn circle is a perfect circle, yet geometry proves exact truths about the circle. The number 7 was prime before anyone counted, and would stay prime if everyone forgot it. Mathematicians often say they discover results rather than invent them.

Mathematical Platonism is the modern view that numbers, sets and other mathematical objects exist independently of minds and of the physical world, and that mathematical statements are true because they describe them. Gottlob Frege and Kurt Gödel both held versions of it. The standard objection, pressed by Paul Benacerraf in 1973, is Plato’s old problem in new clothes: if these objects are outside space and time and cause nothing, how could we ever come to know about them?

Why does Platonic idealism still matter?

Whenever someone says that mathematics discovers truths, that justice is a standard for judging laws and not just whatever the laws say, or that beauty is more than personal taste, they are standing on Plato’s side of an argument that is still open. His idealism was the first rigorous version of a conviction that has never gone away: that the visible world does not explain itself, and that understanding means turning from the shadows to what casts them.

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A note on quotations: translations and paraphrases are our own. For academic work, cite a standard edition using the Stephanus numbers (such as Republic 596a), which are the same in every translation.