I. Plato's Three Realms.—Plato united in his system that which was valuable in the systems of philosophy which preceded his. We may therefore begin immediately with Plato in our preliminary sketch of Greek philosophy.
Plato divides existence into two great realms: that which can be felt by the senses, the "sensible," "to aistheton," and that which can be understood, the "knowable," "to noeton." The most cursory examination of the sense-world reveals the problem of the One and the Many: for every object is one, inasmuch as it is an object, yet manifold in its qualities. Which of these is the most fundamental distinction? Earlier Greek philosophy had given various answers to this question; but none of their conclusions satisfied Plato wholly. Being, "ousia," as such, could not, thought he, be attributed to any finite thing; on the contrary, "becoming," "genesis," was a fitting description of the phenomenal world. He proceeded further to reduce this distinction to its Pythagorean terms, the Limited and the Unlimited. As both of these conceptions are united in that of a definite number, so the truth of both the categories of the One and the Many is their unity, their mixture, which fittingly represented the eternal process of Becoming which may be witnessed in the phenomenal world.
Unity will apply fittingly to the intelligible world, which alone has true Being, being "existing being," "reason," and "existence," "Ontos on, Logos, Ousia."
The Manifold, on the contrary, must apply to the formless, odorless, chaotic matter, "hule," of which the world was formed. We thus reach a third realm of existence, which, however, can only be distinguished as having existed before the creation or formation of the phenomenal world.
Plato thus recognizes three realms of existence: "that which becomes (the sense-world), that in which it becomes (matter), and that from which it is copied (the intelligible world)." God, is the Father, the reason, the "whence it grows," the "hothen phuetai," of the world; matter is the mother and nurse, the concomitant cause, the "En ho gignetai to gignomenon," of the world; and thus the world is the offspring of God and Matter.
But we must not fail to analyze this intelligible world, this "knowable," to noeton." The phrase given above, "that from which it is copied" implies that somebody copies something: that the Deity copies the Ideas or archetypes. There is then, above the intelligible world proper a still higher realm of existence, the Deity: which, in the Pythagorean terminology adduced above would be the Mind, the "Nous," the principle or "cause," "aition," of the phenomenal world.
We have thus four realms of existence: the Deity, the world of Ideas, the world of Sense, and Matter. But as the latter realm has ceased to exist since the creation of the phenomenal world as such, there remain three realms of existence, which are sometimes referred to as the Platonic Trinity: "Nous" or the Deity, the intelligible world of Ideas, and the Sense-world the "sensible," "to aisthetikon." How loose and inaccurate such an appellation is, is clear from the fact that Plato himself did not recognize it. The Sense-world, the supposititious third member of the Trinity, is the only-begotten Son, "Huios monogenes" of the Deity, the "Eikon tou Theou," "Zoon aidion kai noeton," it is a "second" God, "future" before its genesis, and "created" after it; a "blessed deity."
As the world of Ideas is a "Zoon aidion kai noeton," an "eternal and intelligible organism," so the world of Sense is a "Zoon ennoun," an "intelligible organism," a reasonable living being, the creating principle of "Nous," Reason, having reduced the chaotic, necessary, and "alogos," irrational Matter to an image of the world of Ideas.
Thus the problem of the One and the Many was apparently solved: every object being One, in view of its similarity to the Idea according to which, as a pattern, it has been created; and Manifold, in view of the formless matter which had been the condition of its origination.