Possible Problems with Plato's Theory of Forms

Instructor: Dr. Jan Garrett

This page was last updated November 5, 2001.

Plato maintains that

(1) the forms are presuppositions of all meaningful discourse (Cf. Plato, Parmenides 135b-c).

(2) the forms are definite, stable, knowable objects--they are the realities the wise person knows (books V-VIII of the Republic).

Plato also appears to hold the following views concerning forms:

(1) Things become F by participating in the form the F itself. (Things become just by participating in the form the just itself or justice.)

(2) The form for a given attribute F is perfectly F--it has no deficiency. (Self-Predication Assumption.) See Symposium 210e ff.: the beautiful itself, or beauty, is perfectly beautiful.

(3) Things that are F are distinct from the form that makes them F. Equal things are distinct from the form the equal itself. Beautiful things are distinct from the form the Beautiful itself. (Non-identity Assumption. See Parmenides 130b.)

(4) For every (?) set of F things, there is one and only one form (the F itself) because of which they are F. (One-Over-Many Assumption. See Republic 507b)

(5) The Forms, being reality itself, exist by themselves, apart, and do not depend upon particulars. (Parmenides 133c, perhaps a stronger version of Non-Identity)

The Population Problem:

What terms exactly correspond to Platonic Forms?

Plato takes this up in the Parmenides (130b-d). There a "young Socrates" admits that there are forms of likeness, unity, plurality, rightness, beauty, and goodness. He denies that there are any forms for hair, mud, and dirt ("the things are just the things we see" [130d]). He's not sure whether there are forms for human being or for fire and water (130c).

The "Third Man" Argument

(set forth in terms of "large" in the Parmenides [132a-b]). 1) Let a, b, c, d be large things.

2) They are large because of one form, the large itself (L). (One-Over-Many)

3) But L must be distinct from a, b, c, d. (Non-Identity)

4) Now, L must be large. (Self-Predication)

5) What makes L large, along with a, b, c, and d, must be a form too. (One-Over-Many)

6) But this form is distinct from a, b, c, d, and L. Call it L'. (Non-Identity)

7) But L' is large too. (Self-Predication)

and the argument (2-4, 5-7) can be repeated without end.

This is unacceptable because the forms must be something definite if they are to be grasped by the mind, and this infinite series cannot be grasped by the mind.