1. The “Imperfection Argument” (Phaedo 74-76)

    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

    1. We never experience (in sense-perception) objects that are really, precisely, equal, and
    2. We must already have the concept of equality in order to judge the things we encounter in sense-perception to be approximately, imperfectly, equal.

    The argument can be schematized as follows:

    1. We perceive sensible objects to be F.
    2. But every sensible object is, at best, imperfectly F. That is, it is both F and not F (in some respect - shades of Heraclitus??). It falls short of being perfectly F.
    3. We are aware of this imperfection in the objects of perception.
    4. So we perceive objects to be imperfectly F.
    5. To perceive something as imperfectly F, one must have in mind something that is perfectly F, something that the imperfectly F things fall short of. (E.g., we have an idea of equality that all sticks, stones, etc., only imperfectly exemplify.)
    6. So we have in mind something that is perfectly F.
    7. Thus, there is something that is perfectly F (e.g., Equality), that we have in mind in such cases.
    8. Therefore, there is such a thing as the F itself (e.g., the Equal itself), and it is distinct from any sensible object.

  2. A contemporary analogue

    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 don’t 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?

  3. Evaluation of Plato’s argument

    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.

    1. It leaves open the possibility that we are so constituted as to have the concept of equality - i.e., that it is wired in to our brains (or whatever). This would mean that we acquire it at birth. Plato closes this possibility off at 76d1-3: he must think it obvious that we don’t have the concept at birth (since we can’t use it then). We lose it at birth and regain it later, via experience. But he does not establish that we lack the concept at birth (we can’t use it, but that doesn’t mean we don’t have it).

      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.

      Plato’s 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 Plato’s:

      1. I judge myself to be imperfect.
      2. That is, I judge myself to lack perfection.
      3. Hence, I have the idea of perfection.
      4. Hence, there is a being that this idea is an idea of.

        [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).]

    2. A second flaw in the argument:

      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 can’t be reminded of what I’ve 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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