This subject is well treated by Brentano, in his Dissertation Ueber die Bedeutung des Seienden im Aristoteles. See pp. 49-50 seq., of that work. Aristotle observes truly, that these most general terms are the most convenient hiding-places for equivocal meaning (Anal. Post. ii. 97, b. 29). The analogical varieties of Ens or Essence are graduated, according to Aristotle: Complete, Proper, typical, [Greek: ou)si/a], stands at the head: there are then other varieties more or less approaching to this proper type: some of them which [Greek: mikro\n ê)\ ou)the\n e)/chei tou= o)/ntos.] (Metaphys. vi. 1029, b. 9.)] [Side-note: 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. [Footnote 111: Plato, Sophist. p. 250 E.] [Side-note: Plato's views about Non-Ens examined.]