Different definitions of Ens — by Plato — the Materialists, the Idealists.
This enquiry respecting Ens is left by Plato professedly unsettled; according to his very frequent practice. He pretends only to have brought it to this point: that Ens or the Existent is shown to present as many difficulties and perplexities as Non-Ens or the non-existent.[111] I do not think that he has shown thus much; for, according to his definition, Non-Ens is an impossibility: the term is absolutely unmeaning: it is equivalent to the Unknowable or Inconceivable — as Parmenides affirmed it to be. But he has undoubtedly shown that Ens is in itself perplexing: which, instead of lightening the difficulties about Non-Ens, aggravates them: for all the difficulties about Ens must be solved, before you can pretend to understand Non-Ens. Plato has shown that Ens is used in three different meanings:—
1. According to the Materialists, it means only the concrete and particular, including all the attributes thereof, essential and accidental.
2. According to the Idealists or friends of Forms, it means only Universals, Forms, and Attributes.
3. According to Plato’s own definition here given, it means both the one and the other: whatever the mind can either perceive or conceive: whatever can act upon the mind in any way, or for any time however short. It is therefore wholly relative to the mind: yet not exclusively to the perceiving mind (as the Materialists said), nor exclusively to the conceiving mind (as the friends of Forms said): but to both alike.
[111] Plato, Sophist. p. 250 E.
Plato’s views about Non-Ens examined.
Here is much confusion, partly real but principally verbal, about Ens. Plato proceeds to affirm, that the difficulty about Non-Ens is no greater, and that it admits of being elucidated. The higher Genera or Forms (he says) are such that some of them will combine or enter into communion with each other, wholly or partially, others will not, but are reciprocally exclusive. Motion and Rest will not enter into communion, but mutually exclude each other: neither of them can be predicated of the other. But each or both of them will enter into communion with Existence, which latter may be predicated of both. Here are three Genera or Forms: motion, rest, and existence. Each of them is the same with itself, and different from the other two. Thus we have two new distinct Forms or Genera — Same and Different — which enter into communion with the preceding three, but are in themselves distinct from them.[112] Accordingly you may say, motion partakes of (or enters into communion with) Diversum, because motion differs from rest: also you may say, motion partakes of Idem, as being identical with itself: but you cannot say, motion is different, motion is the same; because the subject and the predicate are essentially distinct and not identical.[113]
[112] In the Timæus (pp. 35-36-37), Plato declares these three elements — Ταὐτόν, Θάτερον, Οὐσία — to be the three constituent elements of the cosmical soul, and of the human rational soul.
[113] Plato, Sophist. p. 255 B.