This is both an argument for the existence of Forms and an argument for our possession of a priori concepts. Plato bases the argument on the imperfection of sensible objects and our ability to make judgments about those sensible objects. (The Forms are supposed to be the perfect objects that the sensibles only imperfectly approximate).
The argument as given at Phaedo 74-76 concerns the concept of equality, but it could equally well be given with respect to a number of different concepts (any concept that might have some claim to being an a priori concept).
The argument tries to show that we cannot abstract the concept of equality from our sense-experience of objects that are equal. For
The argument can be schematized as follows:
The linguist Noam Chomsky describes what he calls The Argument from the Impoverished Stimulus as a classic rationalist argument. It notes that we classify physical shapes that we experience (written, printed, drawn, etc.) as inexact representations of geometrically perfect regular figures (squares, circles, triangles, etc.). Why dont we classify them as exact representations of irregular figures?
The idea is that our sensory stimuli are impoverished. We never experience perfect squares, circles, triangles, etc. Yet we have these concepts, and we classify things accordingly. How did we acquire these concepts if we have never experienced anything that they (literally) apply to?
The argument has two faces. Plato uses it not only (1) to establish the existence of supra-sensible Forms, but also (2) to establish that we have cognitive contact with them in a prenatal state. But we should separate these two faces of the argument. For one who was convinced that the argument shows that there must be such objects for a priori concepts might well not be convinced that the argument shows that we must have had some disembodied contact with those objects at some time before we were born.
As an argument for prenatal contact with the Forms, the argument has obvious flaws.
In any event, he does not take this possibility seriously. He has no way to meet the (non-Platonic, anti-empiricist) claim that we have the concept wired in at birth, and hence do not have it before birth (or, at any rate, not very much before birth), and then begin to employ it (fumblingly, at first) in our early childhood bouts with sense-perception. [This is rationalism but without ante-natal existence.]
His not taking this kind of rationalist position seriously may be due to some features his own view shares with empiricism.
Affinities with empiricism:
Plato has it that we derive our concept of equality from experience of objects, after all (just as the empiricist does). However, for Plato, the experience is disembodied experience of non-physical objects of contemplation - the Forms.
Affinities with rationalism:
Plato, like the classic 17th C. rationalists, maintains that we do not derive our concepts from sense-experience by abstraction.
Platos argument that our sensory judgments of (near) equality depend on our having the concept of perfect equality, and that our having such a concept depends upon there being such a thing as perfect equality (i.e., the Form The Equal Itself) for our concept to be a concept of, bears a striking similarity to other rationalist arguments. Thus, cf. Descartes (Meditations III, HR I, 166):
For how would it be possible that I should know . . . that something is lacking in me, and that I am not quite perfect, unless I had within me some idea of a Being more perfect than myself, in comparison with which I should recognize the deficiencies of my nature?
The structure of the argument is the same as Platos:
[There is an added wrinkle in Descartes case: (4) is not deduced directly from (3) but rather by appeal to the principle of the reality of that of which we have a clear and distinct idea and the claim that our idea of perfection is clear and distinct. Plato echoes Parmenides in deducing (4) from (3).]
Socrates notes that in the case of sensible equals, you see their imperfection - their falling short of Equality Itself, which they strive to be like. The sensible equals, nevertheless, make you think of (ennoein, 74d1, lit. put you in mind of) Equality itself. And this must be a case of recollection, says Socrates.
But making one think of or putting one in mind of is not the same as, and does not entail, reminding one of. A 2:15 marathon puts me in mind of, i.e., gives me the idea of, a 1:30 marathon, but it hardly reminds me of it. I cant be reminded of what Ive never experienced, but I certainly can be put in mind of such a thing. At least, Plato has no right to assume the contrary.
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