individual books are transitory, impermanent, unreal; the idea of book is permanent, eternal, it alone has reality; and all individual things, therefore, exist, so far as they have any real existence, only by partaking of the ideas. So all things come into being and owe their existence to sharing in the eternal ideas.” We should be unjust to Plato if we thought that he regarded this doctrine as a perfect explanation of the relation between the visible and invisible worlds. Far from holding such a view he himself evidently held it to be a “guess at truth,” which served to show in its way that there is a permanent reality behind the phenomena of the visible world and a truth which is beyond sense. Indeed Plato is very conscious of troublesome questions which arise in connection with the doctrine, and in three dialogues—his Philebus, Parmenides, and Sophist—he endeavors to meet some of these questions, and there he offers admirable criticism of his own views. With reference to the source of the doctrine, as I have said above, we cannot tell how much Plato derived from Socrates or how much he developed for himself. Socrates was evidently always searching for universals, trying to determine what goodness is in itself in contrast to the goodness embodied in a good man, what are virtue, courage, and such qualities. The teacher or the pupil may have extended the ideas, the universals, to include all things, even the humble articles of furniture which are the examples in the Republic. But in any case by this doctrine of “ideas,” “forms,” Plato secured a basis for reality, a means of attaining absolute knowledge in contrast to that relative knowledge, which according to the Sophists was the utmost which man could attain. Plato would have quite agreed with Protagoras that if the senses were our only avenues to knowledge, then indeed man would be the measure of all things and his knowledge would be limited to the transient phenomenal world; that is, he could have no knowledge of reality; but by apprehending through our reason the ideas—that is, the realities—on which the phenomenal world depends, we can gain genuine knowledge and free ourselves from subjection to mere opinion. Plato also teaches that there are various grades of ideas, some being subordinate to others; the highest of all is that of the Good, identified by him with the Beautiful. This supreme idea is at once the cause of all existence and knowledge, and comprehends within itself all other ideas; as the sun in the visible world, so in the world of true knowledge the Good “is the universal author of all things right and beautiful, itself the source of truth and intelligence.” It is the Absolute, the universal Reason, God.[190]