Metaphysical Form in Ancient and Medieval Thought

Still, these individualized forms, which Plato (c. 428–348 or 347 B.C.E.) briefly recognizes in his Phaedo (102d–103c), are not what he would call Forms or Ideas in his Theory of Forms. Plato's Forms are rather the universal exemplars after which individualized forms are modeled. Consider, for example, any geometrical shape, say, a sphere. Any spherical thing, such as a pearl, is spherical on account of its own spherical shape (its individualized shape), but all pearls (and all billiard balls, bowling balls, etc.) are spherical because they all have the same sort of shape, as if each were just a copy or imitation of a common, universal model, the Form of Sphere, or Sphericity itself. To be sure, different spherical things may realize their common Form differently, say, with different diameters, and with different sorts of imperfections, but insofar as they all realize the same Form, they all constitute the same sort of objects. Indeed, imitating or participating in the same Form is precisely what Plato would take to be the reason why distinct particulars of the same kind belong to the same kind.

But how does one know about these Forms? In his Phaedo (73c.–75c.), Plato presents an interesting argument to show that the ability to recognize things as more or less perfect realizations of their exemplars entails people's souls' prenatal acquaintance with these exemplars. The gist of the argument can be restated as follows. Whenever one sees things that are more or less equal, the ability to recognize them as such and to judge them as being more or less perfectly equal presupposes the acquaintance with absolute, perfect Equality, the Form that all imperfectly equal things are trying to imitate with their imperfect equality. For how else would one know that an equal pair of sticks is not perfectly equal? But perfect equality can certainly never be met in one's sensory experiences. So, this acquaintance with Equality itself cannot be obtained from sensory experience. However, experiences begin with birth. Therefore, people must have their acquaintance with Equality from a prenatal form of existence, from before their souls entered their bodies at birth.

This little piece of reasoning contains all major elements of Plato's philosophical theory in a nutshell. Metaphysically, Forms are the independently existing perfect, universal standards for the perfection of any thing of any given kind. In epistemology, Forms are the source of the possibility of true universal knowledge: by recollecting one's knowledge of their universal exemplar, one has universal knowledge of all particulars that share in the same Form. Finally, in moral and political theory, the realm of Forms is that domain of pure perfection where immortal human souls belong by nature; therefore, one's task in this life is to prepare the soul for its safe return by living one's life according to the standards of perfection set by the Forms.

That the "naïve" Theory of Forms (as presented here) is inconsistent was recognized already by Plato in his Parmenides, where can be found the first formulation of the famous Third Man argument (132a–b), proving the inconsistency of the theory.

Consider the Form of Humanity. According to the theory, each human belongs to this species by participating in one and the same Form. But the Form itself is perfectly human, so it also belongs to the species of humans. Therefore, it should also pertain to this species by participating in the Form of Humanity. However, it cannot participate in itself, for what participates is inferior to what it participates in, and nothing can be inferior to itself. So, there has to be another Form of Humanity, which itself would also have to be human, that is, there would have to be a Third Man, besides the particular humans, and the first Form of Humanity. Indeed, the same reasoning could be repeated, yielding an infinite series of Forms for the species of humans. However, the theory also claims that for a species of particulars there is only a single Form in which they all participate, which is inconsistent with the infinity of Forms also implied by the claims of the theory. Therefore, the theory cannot be true as stated.