I've already consulted four different textbooks on the history of (ancient) philisophy (the ones by Russell, Giovanni Reale/Dario Antiseri, Sir Anthony Kenny and the one I use in high school), as well as Russell's The problems of philosophy and some articles on SEP.

Nonetheless, I still don't completely understand Plato's theory of Forms. I have a feel that all of those books require some prior knowledge about the topic and so leave out lots of details.

This is, roughly, what I don't understand:

  1. I don't get what an idea/form actually is. I know that an idea can be attributed to a class of particulars which share some properties, but what is the idea of that?

  2. What are the arguments in favour of the existence of ideas?

  3. Which philosophical problems does the theory solve?

  4. Are there modern philosophers who support the theory?

    Can you suggest a book, or a series of books, which answers these questions in a rigorously logical, complete and "self-contained" way?

  • 4
    First of all, you could of course read Plato itself. For example the allegory of the cave, in book VII of the Republic (514A and further), provides a very good overview. In any case, please indicate what it is exactly you don't understand, and then people here may be able to explain it to you. If it indeed requires a lot of background knowledge it may require too long an answer for the SE format, but then we can gear our answers towards your situation.
    – user2953
    Apr 12, 2015 at 10:09
  • 2
    You can see Norman Gulley, Plato's Theory of Knowledge (1986) and Gail Fine, On Ideas : Aristotle's Criticism of Plato's Theory of Forms (1993). Apr 12, 2015 at 10:13
  • I don't know if it's a good idea to read Plato directly at my present stage of learning: I fear that, without a background, I would not understand -or worse, misinterpret- most of what he wrote.
    – Adrian
    Apr 12, 2015 at 10:24
  • Thanks for your edit. At the time I used a Dutch textbook, Het oog in de storm, that had zero prior knowledge required, and explained it all fairly well. I don't know of anything in English, unfortunately. I do think reading the text itself wouldn't be a huge problem though: Plato writes very comprehensibly. Still, he won't give you the answer to question 4 :)
    – user2953
    Apr 12, 2015 at 10:34
  • 1
    @Keelan I think I get where Nicol is coming from. While Plato is readable, he isn't particularly convincing, and comes off as someone who doesn't understand epistemology. It's very hard to take Plato's philosophy of forms seriously when one has a better grounding in modern psychology, math and logic. However, given how Plato is still read and re-read, one then starts to wonder if maybe one missed the point, but since Plato's writings didn't cut it, a more modern philosopher with a rigorous take might explain matters better. This is at least my interpretation of Nicole's question.
    – R. Barzell
    Apr 14, 2015 at 20:22

4 Answers 4


In my personal opinion, the "Theory of the Forms" isn't a theory at all, but an extended metaphor (for the Ideal of the Good), which would explain its many inconsistencies. (I'd further venture that everything in Plato should be read in this manner).

With that said, the current version of the Wikipedia article on the subject has a good summary of the Theory as commonly understood, and covers most of what you're looking for, with the exception of modern philosophers who support the theory. On that last, I'd say that everyone has been influenced by the theory, but no one supports it.

To briefly summarize the Wikipedia article's answers to your questions:

  • 1:
    A Form is an abstract, eternal, idealized version of something which exists in the material world. The idea is that the physical world we inhabit is made up of debased and corrupted copies of the Forms --a window is a corrupted copy of a perfect square, for instance. The Forms exist outside time and space, and cannot be directly perceived (except through the mind).

  • 2 & 3:
    (A) - When we perceive different things that have a unified trait (blue jeans and blue sky in the Wikipedia example) we intuit they are both partaking in a more abstract and unified concept. The Theory of Forms thus explains how we can generalize from the particular to the universal. (B) - When we describe something like a wheel as a circle, even though it isn't a perfect circle, we are comparing it mentally to an ideal, and judging it as an imperfect copy of that ideal. The Theory of Forms explains how we can think of things that are perfect without ever having seen things that are perfect.

It's worth noting that while the Theory of Forms itself isn't much of a live concern, it paved the way for all subsequent consideration of things such as concepts, abstract thought, the soul, universals, exemplification and so forth.


There's a lot going on in here that relates to the history of thought. We get our English word idea from the Greek word ἰδέα ( idea in their alphabet). But we often use it like "I have an idea". We assume ideas are things in our head.

  1. For Plato (at least middle), ideas are not things in our head but something more real than the things we encounter, which in turn inspires the things we encounter. To give this some concrete weight, think about a cow. I assume you can form an image of a cow. Then ask when you see actual cows, do you think your "idea" comes from the cow or that cows are copying an idea (with idea meaning a form that exists outside of any cow)? Plato's answer is the latter.

  2. There are a lot of different arguments for or against the existence of Forms. The advantage is that they can help explain how we can understand things. The big disadvantage is that we run into problems about (a) how we have these ideas in our head, (b) how many of these ideas exist, (c) what these ideas are.

  3. The point is to solve a problem where what we see in the world does not live up to the ideas. So to give a few examples from Plato, we have dialogues about the nature of piety, justice, and love. These are things we want to be better than some of the examples we might see in the world.

  4. There are not that many philosophers proper who are full fledged Platonists, because it's a pretty hard position to defend. But there are a lot of mathematical Platonists -- people who believe the numbers are real even apart from all of their instances.


In my opinion your question addresses the fundamental aspects of Plato's theory of forms.

ad 1: In my opinion the best introduction to the question "What is a Platonic form?" is Plato's Symposion, section 210ff from Diotima's speech. Here Plato exemplifies the theory of forms by the idea of beauty. In modern terms: Plato shows how to approach an idea by abstraction from concrete examples. It is a bottom-up approach.

ad 2. Plato's answer: We know the forms because we could grasp them in a status of pre-existence (Phaidon 75ff)

ad 3. According to Plato, the theory of forms answers the question: Why do we recognize beautiful things, fair laws, true statements etc.? Because we know a priori the idea of beauty, of justice, of truth.

ad 4. I do not know any important contemporary philosophers who defend Plato's theory of forms.

On the academic level good expositions of Plato's philosophy exist, e.g., "The Oxford Handbook of Plato" contains an essay by Verity Harte on "Plato's metaphysics". She discusses the fundamental topics of the Theory of Forms.

My personal opinion: Plato's theory of forms is the earliest example of a philosophical theory. Its goal is to answer deep questions from the domain which today is named epistemology. Therefore Plato's theory of forms is important from a historical point of view.

But Plato's theory suffers from many severe handicaps: The theory builds on the concept of a pre-existent soul. Hence its premisses are much harder than the original epistemological problem. The ontological status of Plato's forms is unclear. Forms are often treated like separate substances. Sometimes Plato even ascribes the attribute which is represented by the idea to the idea itself, e.g., the idea of beauty is beautiful.

I am willing to reserve for Plato's theory of forms a special room in a virtual museum of European history of ideas, but anyhow: It is a room in a museum!


I have created a series of lecture notes on Plato's theory of the forms, which address many of the issues you are asking about. You can find them at my course website.

I would recommend you look at lectures HN01 and HN04.

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