This may seem, at first sight, a very singular doctrine. That there can be no knowledge of sense-objects would, it might seem to us moderns, involve the denial that modern physical science, with all its exactitude and accumulated knowledge, is knowledge at all. And surely, though all earthly things arise and pass away, many of them last long enough to admit of knowledge. Surely the mountains are sufficiently permanent to allow us to know something of them. They have relative, though not absolute, permanence. This criticism is partly justified. Plato did underestimate the value of physical knowledge. But for the most part, the criticism is a misunderstanding. By the world of sense Plato means bare sensation with no rational element in it. Now physical science has not such crude sensation for its object. Its objects are rationalized sensations. [{193}] If, in Plato's manner, we think only of pure sensation, then it is true that it is nothing but a constant flux without stability; and knowledge of it is impossible. The mountains are comparatively permanent. But our sensation of the mountains is perpetually changing. Every change of light, every cloud that passes over the sun, changes the colours and the shades. Every time we move from one situation to another, the mountain appears a different shape. The permanence of the mountain itself is due to the fact that all these varying sensations are identified as sensations of one and the same object. The idea of identity is involved here, and it is, as it were, a thread upon which these fleeting sensations are strung. But the idea of identity cannot be obtained from the senses. It is introduced into things by reason. Hence knowledge of this permanent mountain is only possible through the exercise of reason. In Plato's language, all we can know of the mountain is the Ideas in which it participates. To revert to a previous example, even the knowledge "this paper is white" involves the activity of intellect, and is impossible through sensation alone. Bare sensation is a flow, of which no knowledge is possible.
Aristotle observes that Plato's theory of Ideas has three sources, the teachings of the Eleatics, of Heracleitus, and of Socrates. From Heracleitus, Plato took the notion of a sphere of Becoming, and it appears in his system as the world of sense. From the Eleatics he took the idea of a sphere of absolute Being. From Socrates he took the doctrine of concepts, and proceeded to identify the Eleatic Being with the Socratic concepts. This gives him his theory of Ideas.
Sense objects, so far as they are knowable, that is so far as they are more than bare sensations, are so only because the Idea resides in them. And this yields the clue to Plato's teaching regarding the relation of sense objects to the Ideas. The Ideas are, in the first place the cause, that is to say, the ground (not the mechanical cause) of sense-objects. The Ideas are the absolute reality by which individual things must be explained. The being of things flows into them from the Ideas. They are "copies," "imitations," of the Ideas. In so far as they resemble the Idea, they are real; in so far as they differ from it, they are unreal. In general, sense objects are, in Plato's opinion, only very dim, poor and imperfect copies of the Ideas. They are mere shadows, and half-realities. Another expression frequently used by Plato to express this relationship is that of "participation." Things participate in the Ideas. White objects participate in the one whiteness, beautiful objects, in the one beauty. In this way beauty itself is the cause or explanation of beautiful objects, and so of all other Ideas. The Ideas are thus both transcendent and immanent; immanent in so far as they reside in the things of sense, transcendent inasmuch as they have a reality of their own apart from the objects of sense which participate in them. The Idea of man would still be real even if all men were destroyed, and it was real before any man existed, if there ever was such a time. For the Ideas, being timeless, cannot be real now and not then.
Of what kinds of things are there Ideas? That there are moral Ideas, such as Justice, Goodness, and Beauty, Ideas of corporeal things, such as horse, man, tree, star, river, and Ideas of qualities, such as whiteness, heaviness, [{195}] sweetness, we have already seen. But there are Ideas not only of natural corporeal objects, but likewise of manufactured articles; there are Ideas of beds, tables, clothes. And there are Ideas not only of exalted moral entities, such as Beauty and Justice. There are also the Ideal Ugliness, and the Ideal Injustice. There are even Ideas of the positively nauseating, such as hair, filth, and dirt. This is asserted in the "Parmenides." In that dialogue Plato's teaching is put into the mouth of Parmenides. He questions the young Socrates whether there are Ideas of hair, filth, and dirt. Socrates denies that there can be Ideas of such base things. But Parmenides corrects him, and tells him that, when he has attained the highest philosophy, he will no longer despise such things. Moreover, these Ideas of base things are just as much perfection in their kind as Beauty and Goodness are in theirs. In general, the principle is that there must be an Idea wherever a concept can be formed; that is, wherever there is a class of many things called by one name.
We saw, in treating of the Eleatics, that for them the absolute Being contained no not-being, and the absolute One no multiplicity. And it was just because they denied all not-being and multiplicity of the absolute reality that they were unable to explain the world of existence, and were forced to deny it altogether. The same problem arises for Plato. Is Being absolutely excludent of not-being? Is the Absolute an abstract One, utterly exclusive of the many? Is his philosophy a pure monism? Is it a pluralism? Or is it a combination of the two? These questions are discussed in the "Sophist" and the "Parmenides."
Plato investigates the relations of the One and the many, Being and not-being, quite in the abstract. He decides the principles involved, and leaves it to the reader to apply them to the theory of Ideas. Whether the Absolute is one or many, Being or not-being, can be decided independently of any particular theory of the nature of the Absolute, and therefore independently of Plato's own theory, which was that the Absolute consists of Ideas. Plato does not accept the Eleatic abstraction. The One cannot be simply one, for every unity must necessarily be a multiplicity. The many and the One are correlative ideas which involve each other. Neither is thinkable without the other. A One which is not many is as absurd an abstraction as a whole which has no parts. For the One can only be defined as that which is not many, and the many can only be defined as the not-one. The One is unthinkable except as standing out against a background of the many. The idea of the One therefore involves the idea of the many, and cannot be thought without it. Moreover, an abstract One is unthinkable and unknowable, because all thought and knowledge consist in applying predicates to subjects, and all predication involves the duality of its subject.
Consider the simplest affirmation that can be made about the One, namely, "The One is." Here we have two things, "the One," and "is," that is to say, being. The proposition means that the One is Being. Hence the One is two. Firstly, it is itself, "One." Secondly, it is "Being," and the proposition affirms that these two things are one. Similarly with any other predicate we apply to the One. Whatever we say of it involves its duality. Thus we find that all systems of thought which [{197}] postulate an abstract unity as ultimate reality, such as Eleaticism, Hinduism, and the system of Spinoza, attempt to avoid the difficulty by saying nothing positive about the One. They apply to it only negative predicates, which tell us not what it is, but what it is not. Thus the Hindus speak of Brahman as formless, immutable, imperishable, unmoved, uncreated. But this, of course, is a futile expedient. In the first place, even a negative predicate involves the duality of the subject. And, in the second place, a negative predicate is always, by implication, a positive one. You cannot have a negative without a positive. To deny one thing is to affirm its opposite. To deny motion of the One, by calling it the unmoved, is to affirm rest of it. Thus a One which is not also a many is unthinkable. Similarly, the idea of the many is inconceivable without the idea of the One. For the many is many ones. Hence the One and the many cannot be separated in the Eleatic manner. Every unity must be a unity of the many. And every many is ipso facto a unity, since we think the many in one idea, and, if we did not, we should not even know that it is a many. The Absolute must therefore be neither an abstract One, nor an abstract many. It must be a many in one.
Similarly, Being cannot totally exclude not-being. They are, just as much as the One and the many, correlatives, which mutually involve each other. The being of anything is the not-being of its opposite. The being of light is the not-being of darkness. All being, therefore, has not-being in it.