But at this point, perhaps, a glimmer of hope dawns upon us. We ask the reason for these Ideas. Has not Plato asserted that the ultimate reason and ground of all the lower Ideas will be found in the supreme Idea of [{244}] the Good? Now if this is so, it means that the lower Ideas must find their necessity in the highest Idea. If we could see that the Idea of the Good necessarily involves the other Ideas, then these other Ideas would be really explained. In other words, we ought to be able to deduce all the other Ideas from this one Idea. It ought to be possible to show that, granted the Idea of the Good, all the other Ideas necessarily follow, that to assume the Good and deny the other Ideas would be self-contradictory and unthinkable. There are examples in Plato of the kind of deduction we require. For example, in the "Parmenides" he showed that the Idea of the one necessarily involves the Idea of the many, and vice versa. You cannot think the one without also thinking the many. This means that the many is deduced from the one, and the one from the many. Just in the same way, we ought to be able to deduce the Idea of whiteness from the Idea of the Good. But this is clearly not possible. You may analyse the Good as long as you like, you may turn it in every conceivable direction, but you cannot get whiteness out of it. The two Ideas do not involve each other. They are thinkable apart. It is quite possible to think the Good without thinking whiteness. And it is the same with all the other Ideas. None of them can be deduced from the Good.
And the reason of this is very obvious. Just as the lower Ideas contain only what is common among the things of a class, and exclude their differences, so the higher Ideas include what is common to the Ideas that come under them, but exclude what is not common. For example, the Idea of colour contains what white, blue, red, and green, have in common. But all colours [{245}] have not whiteness in common. Green, for example, is not white. Hence the Idea of colour excludes the Idea of whiteness, and it likewise excludes all the Ideas of the other particular colours. So too the highest Idea of all contains only what all the Ideas agree in, but all the rest falls outside it. Thus the Idea of whiteness is perfect in its kind. And as all Ideas are likewise perfect, the highest Idea is that in which they all agree, namely, perfection itself. But this means that the perfection of the Idea of whiteness is contained in the supreme Idea, but its specific character in which it differs from other Ideas is excluded. Its specific character is just its whiteness. Thus the perfection of whiteness is contained in the Good, but its whiteness is not. Consequently it is impossible to deduce whiteness from the Good, because the Good does not contain whiteness. You cannot get out of it what is not in it. When Plato deduced the many from the one, he did so only by showing that the One contains the many. He cannot deduce whiteness from goodness, because goodness does not contain whiteness.
The lower Ideas thus have not the character of necessity. They are mere facts. And the hope that we shall find their necessity in the supreme Idea fails. But suppose we waive this. Suppose we grant that there must be an Idea of whiteness, because there is an Idea of the Good. Then why is there an Idea of the Good? What is the necessity of that? We cannot see any necessity in it. What we said of the other Ideas applies with equal force to the highest Idea. The Good may be a necessary Idea, but Plato has not shown it.
Thus, though Plato named reason as the Absolute, [{246}] and though reason is a self-explanatory principle, his account of the detailed content of reason is so unsatisfactory that none of the concepts which he includes in it are really shown to be rational. His philosophy breaks down upon the second test as it did upon the first. He has neither explained the world from the Ideas, nor has he made the Ideas explain themselves.
There is one other defect in Plato's system which is of capital importance. There runs throughout it a confusion between the notions of reality and existence. To distinguish between existence and reality is an essential feature of all idealism. Even if we go back to the dim idealism of the Eleatics, we shall see this. Zeno, we saw, denied motion, multiplicity, and the world of sense. But he did not deny the existence of the world. That is an impossibility. Even if the world is delusion, the delusion exists. What he denied was the reality of existence. But if reality is not existence, what is it? It is Being, replied the Eleatics. But Being does not exist. Whatever exists is this or that particular sort of being. Being itself is not anywhere to be found. Thus the Eleatics first denied that existence is reality, and then that reality exists. They did not themselves draw this conclusion, but it is involved in their whole position.
With a fully developed idealism, like Plato's, this ought to be still clearer. And, in a sense, it is. The individual horse is not real. But it certainly exists. The universal horse is real. But it does not exist. But, upon this last point, Plato wavered and fell. He cannot resist the temptation to think of the absolute reality as existing. And consequently the Ideas are [{247}] not merely thought as the real universal in the world, but as having a separate existence in a world of their own. Plato must have realised what is, in truth, involved in his whole position, that the absolute reality has no existence. For he tells us that it is the universal, and not any particular individual thing. But everything that exists is an individual thing. Again, he tells us that the Idea is outside time. But whatever exists must exist at some time. Here then this central idealistic thought seems well fixed in Plato's mind. But when he goes on to speak of recollection and reincarnation, when he tells us that the soul before birth dwelt apart in the world of Ideas, to which after death it may hope to return, it is clear that Plato has forgotten his own philosophy, that he is now thinking of the Ideas as individual existences in a world of their own. This is a world of Ideas having a separate existence and place of its own. It is not this world. It is a world beyond. Thus the Platonic philosophy which began on a high level of idealistic thinking, proclaiming the sole reality of the universal, ends by turning the universal itself into nothing but an existent particular. It is the old old story of trying to form mental pictures of that which no picture is adequate to comprehend. Since all pictures are formed out of sensuous materials, and since we can form no picture of anything that is not an individual thing, to form a picture of the universal necessarily means thinking of it as just what it is not, an individual. So Plato commits the greatest sin that can be ascribed to a philosopher. He treats thought as a thing.
To sum up. Plato is the great founder of idealism, the initiator of all subsequent truths in philosophy. [{248}] But, as always with pioneers, his idealism is crude. It cannot explain the world; it cannot explain itself. It cannot even keep true to its own principles, because, having for the first time in history definitely enunciated the truth that reality is the universal, it straightway forgets its own creed and plunges back into a particularism which regards the Ideas as existent individuals. It was these defects which Aristotle set himself to rectify in a purer idealism, shorn of Plato's impurities.
ARISTOTLE