Both in the Sophistes and in the Parmenides, the leading disputant introduced by Plato is not Sokrates, but Parmenides and another person (unnamed) of the Eleatic school. In both dialogues objections are taken against the Realistic theory elsewhere propounded by Plato, though the objections adduced in the one are quite distinct from those noticed in the other. In the Sophistes, the Eleatic reasoner impugns successfully the theories of two classes of philosophers, one the opposite of the other: first, the Materialists, who recognized no Entia except the Percepta of Sense; next, the Realistic Idealists, who refused to recognize these last as real Entia, or as anything more than transient and mutable Generata or Fientia, while they confined the title of Entia to the Forms, cogitable, incorporeal, eternal, immutable, neither acting on anything, nor acted upon by anything. These persons are called in the Sophistes “Friends of Forms,� and their theory is exactly what we have already cited out of so many other dialogues of Plato, drawing the marked line of separation between Entia and Fientia; between the Immutable, which alone is real and cognizable, and the Mutable, neither real nor cognizable. The Eleate in the Sophistes controverts this Platonic theory, and maintains that among the Universal Entia there are included items mutable as well as immutable; that both are real and both cognizable; that Non-Ens (instead of being set in glaring contrast with Ens, as the totally incogitable against the infallibly cognizable)[11] is one among the multiplicity of Real Forms, meaning only what is different from Ens, and therefore cognizable not less than Ens; that Percepta and Cogitata are alike real, yet both only relatively real, correlating with minds percipient and cogitant. Thus, the reasoning in the Sophistes, while it sets aside the doctrine of Universalia ante rem, does not mark out any other relation between Universals and Particulars (neither in re nor post rem). It discusses chiefly the intercommunion or reciprocal exclusion of Universals with respect to each other; and upon this point, far from representing them as objects of infallible Cognition as contrasted with Opinion, it enrolls both Opinion and Discourse among the Universals themselves, and declares both of them to be readily combinable with Non-Ens and Falsehood. So that we have here error and fallibility recognized in the region of Universals, as well as in that of Particulars.
[11] Plato, Republic, v. pp. 478, 479.
But it is principally in the dialogue Parmenides that Plato discusses with dialectical acuteness the relation of Universals to their Particulars; putting aside the intercommunion (affirmed in the Sophistes) or reciprocal exclusion between one Universal and another, as an hypothesis at least supremely difficult to vindicate, if at all admissible.[12] In the dialogue, Sokrates is introduced in the unusual character of a youthful and ardent aspirant in philosophy, defending the Platonic theory of Ideas as we have seen it proclaimed in the Republic and in the Timæus. The veteran Parmenides appears as the opponent to cross-examine him; and not only impugns the theory by several interrogatories which Sokrates cannot answer, but also intimates that there remain behind other objections equally serious requiring answer. Yet at the same time he declares that, unless the theory be admitted, and unless Universalia ante rem can be sustained as existent, there is no trustworthy cognition attainable, nor any end to be served by philosophical debate. Moreover, Parmenides warns Sokrates that, before he can acquire a mental condition competent to defend the theory, he must go through numerous preliminary dialectical exercises; following out both the affirmative and the negative hypotheses in respect to a great variety of Universals severally. To illustrate the course prescribed, Parmenides gives a long specimen of this dialectic in handling his own doctrine of Ens Unum. He takes first the hypothesis Si Unum est, next the hypothesis Si Unum non est; and he deduces from each, by ingenious subtleties, double and contradictory conclusions. These he sums up at the end, challenging Sokrates to solve the puzzles before affirming his thesis.
[12] Plato, Parmenid. p. 129, E.; with Stallbaum’s Prolegomena to that dialogue, pp. 38-42.
Apart from these antinomies at the close of the dialogue, the cross-examination of Sokrates by Parmenides, in the middle of it, brings out forcibly against the Realistic theory objections such as those urged against it by the Nominalists of the Middle Ages. In the first place, we find that Plato conceived the theory itself differently from Porphyry and the philosophers that wrote subsequently to the Peripatetic criticism. Porphyry and his successors put the question, Whether Genera and Species had a separate existence, apart from the Individuals composing them? Now, the world of Forms (the Cogitable or Ideal world as opposed to the Sensible) is not here conceived by Plato as peopled in the first instance by Genera and Species. Its first tenants are Attributes, and attributes distinctly relative — Likeness, One and Many, Justice, Beauty, Goodness, &c. Sokrates, being asked by Parmenides whether he admits Forms corresponding with these names, answers unhesitatingly in the affirmative. He is next asked whether he admits forms corresponding to the names Man, Fire, Water, &c., and, instead of replying in the affirmative, intimates that he does not feel sure. Lastly, the question is put whether there are Forms corresponding to the names of mean objects — Mud, Hair, Dirt, &c. At first he answers emphatically in the negative, and treats the affirmative as preposterous; there exist no cogitable Hair, &c., but only the object of sense that we so denominate. Yet, on second thoughts, he is not without misgiving that there may be Forms even of these; though the supposition is so repulsive to him that he shakes it off as much as he can. Upon this last expression of sentiment Parmenides comments, ascribing it to the juvenility of Sokrates, and intimating that, when Sokrates has become more deeply imbued with philosophy, he will cease to set aside any of these objects as unworthy.
Here we see that, in the theory of Realism as conceived by Sokrates, the Self-Existent Universals are not Genera and Species as such, but Attributes — not Second Substances or Essences, but Accidents or Attributes, e.g. Quality, Quantity, Relation, &c., to use the language afterwards introduced in the Aristotelian Categories; that no Genera or Species are admitted except with hesitation; and that the mean and undignified among them are scarcely admissible at all. This sentiment of dignity, associated with the Universalia ante rem, and emotional necessity for tracing back particulars to an august and respected origin, is to be noted as a marked and lasting feature of the Realistic creed; and it even passed on to the Universalia in re, as afterwards affirmed by Aristotle. Parmenides here takes exception to it (and so does Plato elsewhere[13]) as inconsistent with faithful adherence to scientific analogy.
[13] Plato, Sophist. p. 227, A. Politikus, p. 266, D.
Parmenides then proceeds (interrogating Sokrates) first to state what the Realistic theory is (Universals apart from Particulars — Particulars apart from Universals, yet having some participation in them, and named after them), next to bring out the difficulties attaching to it. The Universal or Form (he argues) cannot be entire in each of its many separate particulars; nor yet is it divisible, so that a part can be in one particular, and a part in another. For take the Forms Great, Equal, Small; Equal magnitudes are equal because they partake in the Form of Equality. But how can a part of the Form Equality, less than the whole Form, cause the magnitudes to be equal? How can the Form Smallness have any parts less than itself, or how can it be greater than anything?
The Form cannot be divided, nor can it co-exist undivided in each separate particular; accordingly, particulars can have no participation in it at all.
Again, you assume a Form of Greatness, because you see many particular objects, each of which appears to you great; this being the point of resemblance between them. But if you compare the Form of Greatness with any or all of the particular great objects, you will perceive a resemblance between them; this will require you to assume a higher Form, and so on upward without limit.