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. [Side-note: The argument of Plato goes to an entire denial of the Absolute, and a full establishment of the Relative.]