Let us then first examine the central thought of Plato’s philosophy—the “doctrine of ideas.” Developing the doctrines of earlier philosophers, especially those of Heraclitus and Anaxagoras, Plato held that the world is dual. In it is the phenomenal world visible to us, which includes all natural objects and those made by man, a transient and unreal world which we know only through our senses. But beside it, or rather behind it, is another world invisible but permanent and real, which can be grasped only by the reason. This is the world of ideas. Yet the two worlds are not separate, for the world of the senses owes its existence wholly to its dependence on the world of ideas. To understand Plato’s view we must consider in an elementary way what he meant by “ideas.” The words which he uses (εἶδος, ἰδέα) signify “form,” and in logic are used in the sense of “class,” “kind,” “species,” “the general principle for the classification of objects.” The translation “idea” is traditional, and there is no adequate reason for preferring “form” or any other English equivalent. Now Plato’s statement that the world of phenomena depends on the invisible world of ideas seems at first sight paradoxical, for by it he means that the individual tree, book, desk, chair, good man, or whatever you please, is not the real being at all, but that the ideas of tree, book, desk, chair, and goodness alone possess reality. It may be made plain by an illustration which shall be Plato’s own. At the beginning of the tenth book of the Republic Socrates and Glaucon are conversing together. The master wins Glaucon’s assent first to the proposition that although there are many beds and tables in the world, there are only two ideas, one of a bed, the other of a table. He then goes on to show that the workman makes a bed or table by shaping his material according to the idea of a bed or a table, but that he does not create the ideas themselves. That is done by God who is the real maker of the real bed, that is, of the idea of a bed. The carpenter makes only the particular bed which owes its temporary existence to the eternal idea—the real bed—which is in Nature, in the mind of God. Or if Plato should appear before us tonight, he might say, “Suppose we take a dozen books of different sizes and different shapes and appearance, how do we recognize that these diverse objects are all books?” Then when we hesitated to give an answer, as we probably should, he would reply, “It is because each one of these individual books partakes of the idea of book. The idea is present in the individual example and thereby gives the individual its existence; the individual depends therefore on the idea, not the idea on the individual. If this dozen, or indeed if all the books in the entire world were to be destroyed, the idea of book would still remain, and new books could be made by causing the materials, out of which books are constructed, to partake of the idea of book. That is, all