From what I understand about Platos theory of Forms, it seems little to do with mathematical concepts, although he uses geometry as an example, for example the form of the Triangle, he also introduces other Forms, such as the Good. It appears that what abstract mathematical entities live are and where they live is not his main concern here.

In the philosophy of mathematics one is introduced to the Platonic realm where mathematical concepts like the real line and the number one lives - but nothing else lives there.

What are the connection between these two ideas? Is it purely historical, in the sense that Platos theory of Forms inspired the one for the philosophy of mathematics. If so, how far can we trace this idea as a distinct one in the philosophy of mathematics; or is there a proper relationship between them in the sense the Platonic realm is Platos theory of Forms restricted to mathematics - but then Why were the other Forms left out?

  • I mean, the analogy is from Plato himself (triangles and so on; see e.g., the Meno and Timaeus...) --Maybe you could tell us a little more about what you're looking for an explanation about here? What research have you done and what have you found out so far?
    – Joseph Weissman
    Commented Jun 16, 2013 at 20:50
  • My guess is the following: Plato's theory of forms survived in some form among the neo-platonists in the medieval period during their debates over the existence of universals. Forms were the platonic precursor to Aristotle's universals. Forms/universals were the paradigmatic abstract objects and so the view that mathematics is about abstract objects got dubbed Platonism. This is all just my own speculations, don't really have much to substantiate these claims (hence why I'm posting as a comment).
    – Dennis
    Commented Jun 16, 2013 at 21:12
  • @weissman: ok, I didn't know that Plato used geometry as an example of his forms. Commented Jun 16, 2013 at 22:20
  • Definitely. And in a way it's even more than that; in the Timeaus, the triangular form is even held to have 'cosmogonic' properties. --In passing: Plato's entire aim in a way was to 'educate' philosophers through geometry and geometric reasoning; while it would take a lot of work to substantiate this, in a way it's kind of plain in any encounter with his work (again, the Meno seems like it would be particularly illuminating here; though the triangles do show up in a lot of places...)
    – Joseph Weissman
    Commented Jun 16, 2013 at 23:29
  • @Weissman: That is a large claim! I distinctly recall reading in the Republic when Plato talks about the uses of geometry in education - he says that it offers a glimpse of the eternal, but he also warns that it can narrow the mind. In what way does Plato claim that the triangular form is 'cosmogenic'? And why would the Meno be illuminating? Commented Jun 17, 2013 at 1:26

2 Answers 2


The connection between the idea of the Good and the idea of the Triangle is that, in the physical world, there exist neither things that are perfectly good nor things that are perfect triangles. Even if you help someone, you probably do it because it gives you a pleasant feeling, or because it's just a habit, so it's not pure good. Similarly, the triangle drawn in my book is not a perfect triangle: it is ever so slightly deformed by the structure of the paper and the inaccuracy of whatever drew it.

I could say, "I have this figure here in my book that looks like it has three angles, but it's ever so slightly off; I will refer to it by describing the little quirks in its outline that come closest to its actual shape". Alternatively, I could simply say, "granted, it is an imperfect triangle; but we all know what a perfect triangle would be in theory, and it's more efficient to refer to this perfect triangle when discussing mathematics rather than this imperfect drawing in my book, so I will just call it a triangle, keeping this perfect theoretical triangle in mind". The same can be applied to a good action.

The concept of an ideal, perfect triangle or the perfect good can be helpful and efficient in our daily tasks. But Plato went farther: he held that their perfection makes them in a way divine. They are different from physical triangles and good behaviour in our daily lives; they're something of a higher order. That's why he postulated that these idea(l)s must exist in some way in a higher plane of existence, that they must transcend their imperfect physical copies.

The fact that we, humans, could in some way touch on or even comprehend these ideas must mean that we, too, must be or have been connected with the divine. For how can we have a perfect triangle in mind if we have never seen one? A being in no way connected to the divine surely could not distil a perfect triangle out of the imperfect physical things in the temporal world.


In a nutshell: to promote his political agenda Plato invented a new rhetoric which relies on imitating mathematics. Aristotle was able to 'abstract' logic from discourse while Plato offered mostly analogies.

Geometry does not have a scale, so any circle is the unit circle or all circles are indeed the same. Two circles are already numerically different and, further, they are spatially different (differing by places, and/or size). Same for squares, cubes etc. It is the absence of differences that makes them 'perfect'. What fascinated Plato is the generality of mathematics, e.g. that for any triangle there is a circle passing through its vertices. Analogies are easy to construct: for any man there should be a compelling idea of justice, of a King etc.

The trick does not work smoothly but its weakness was transformed into strength: there is no such things as perfect dirt or greed. Negatives entities are not Good, of course. The exemple offered by Plato in his Republic borders on farce as he entertains (seriously?) the idea of (perfect) bed.

There are a serious difficulties when discussing Plato in English. Just because native speakers take 'ideas' to be mostly subjective, they are called instead 'forms' (a latin word) and for 'forms', such as the ones exhibited by mathematics, a hazy conception about some kind of 'intermediates' is supplied. The famous analogy of the divided line (Rep.) exposes how Plato sought to place his imitation as the pinnacle of philosophy.

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