Plato’s argument against the Idealists or Friends of Forms. Their point of view against him.
Plato now presents this same definition to the opposite class of philosophers: to the Idealists, or partisans of the incorporeal — or of self-existent and separate Forms. These thinkers drew a marked distinction between the Existent and the Generated — between Ens and Fiens — τὸ ὂν and τὸ γιγνόμενον. Ens or the Existent was eternal and unchangeable: Fiens or the Generated was always in change or transit, coming or going. We hold communion (they said) with the generated or transitory, through our bodies and sensible perceptions: we hold communion with unchangeable Ens through our mind and by intellection. They did not admit the definition of existence just given by Plato. They contended that that definition applied only to Fiens or to the sensible world — not to Ens or the intelligible world.[105] Fiens had power to act and be acted upon, and existed only under the condition of being so: that is, its existence was only temporary, conditional, relative: it had no permanent or absolute existence at all. Ens was the real existent, absolute and independent — neither acting upon any thing nor being acted upon. They considered that Plato’s definition was not a definition of Existence, or the Absolute: but rather of Non-Existence, or the Relative.
[105] Plato, Sophist. p. 248 C.
Plato argues — That to know, and be known, is action and passion, a mode of relativity.
But (asks Plato in reply) what do you mean by “the mind holding communion” with the intelligible world? You mean that the mind knows, comprehends, conceives, the intelligible world: or in other words, that the intelligible world (Ens) is known, is comprehended, is conceived, by the mind. To be known or conceived, is to be acted on by the mind.[106] Ens, or the intelligible world, is thus acted upon by the mind, and has a power to be so acted upon: which power is, in Plato’s definition here given, the characteristic mark of existence. Plato thus makes good his definition as applying to Ens, the world of intelligible Forms — not less than to Fiens, the world of sensible phenomena.
[106] Plato, Sophist. p. 248 D. εἰ προσομολογοῦσι τὴν μὲν ψυχὴν γινώσκειν, τὴν δ’ οὐσίαν γιγνώσκεσθαι … Τί δέ; τὸ γινώσκειν ἢ γιγνώσκεσθαι φατὲ ποίημα ἢ πάθος ἢ ἀμφότερον;
The definition of existence, here given by Plato, and the way in which he employs it against the two different sects of philosophers — Materialists and Idealists — deserves some remark.
Plato’s reasoning — compared with the points of view of both.
According to the Idealists or Immaterialists, Plato’s definition of existence would be supposed to establish the case of their opponents the Materialists, who recognised nothing as existing except the sensible world: for Plato’s definition (as the Idealists thought) fitted the sensible world, but fitted nothing else. Now these Idealists did not recognise the sensible world as existent at all. They considered it merely as Fiens, ever appearing and vanishing. The only Existent, in their view, was the intelligible world — Form or Forms, absolute, eternal, unchangeable, but neither visible nor perceivable by any of the other senses. This is the opinion against which Plato here reasons, though in various other dialogues he gives it as his own opinion, or at least, as the opinion of his representative spokesman.
In this portion of the present dialogue (Sophistês) the point which he makes is, to show to the Idealists, or Absolutists, that their Forms are not really absolute, or independent of the mind: that the existence of these forms is relative, just as much as that of the sensible world. The sensible world exists relatively to our senses, really or potentially exercised: the intelligible world exists relatively to our intelligence, really or potentially exercised. In both cases alike, we hold communion with the two worlds: the communion cannot be left out of sight, either in the one case or in the other. The communion is the entire and fundamental fact, of which the Subject conceiving and the Object conceived, form the two opposite but inseparable faces — the concave and convex, to employ a favourite illustration of Aristotle. Subject conceiving, in communion with Object conceived, are one and the same indivisible fact, looked at on different sides. This is, in substance, what Plato urges against those philosophers who asserted the absolute and independent existence of intelligible Forms. Such forms (he says) exist only in communion with, or relatively to, an intelligent mind: they are not absolute, not independent: they are Objects of intelligence to an intelligent Subject, but they are nothing without the Subject, just as the Subject is nothing without them or some other Object. Object of intelligence implies an intelligent Subject: Object of sense implies a sentient Subject. Thus Objects of intelligence, and Objects of sense, exist alike relatively to a Subject — not absolutely or independently.