The Socratic teaching is essentially ethical in character. In this alone did Socrates bear any resemblance to the Sophists. It was the Sophists who had introduced into Greek philosophy the problem of man, and of the duties of man. And to these problems Socrates also turns his exclusive attention. He brushes aside all questions as to the origin of the world, or the nature of the ultimate reality, of which we have heard so much in the philosophies of the earlier thinkers. Socrates openly deprecated such speculations and considered all such knowledge comparatively worthless as against ethical knowledge, the knowledge of man. Mathematics, physics, and astronomy, he thought, were not valuable forms of knowledge. He said that he never went for walks outside the city, because there is nothing to be learnt from fields and trees. Nevertheless the ethical teaching of Socrates was founded upon a theory of knowledge, which is quite simple, but extremely important. The Sophists had founded knowledge upon perception, with the result {143} that all objective standards of truth had been destroyed. It was the work of Socrates to found knowledge upon reason, and thereby to restore to truth its objectivity. Briefly, the theory of Socrates may be summarized by saying that he taught that _all knowledge is knowledge through concepts_. What is a concept? When we are directly conscious of the presence of any particular thing, a man, a tree, a house, or a star, such consciousness is called perception. When, shutting our eyes, we frame a mental picture of such an object, such consciousness is called an image or representation. Such mental images are, like perceptions, always ideas of particular individual objects. But besides these ideas of individual objects, whether through sense-perception or imagination, we have also general ideas, that is to say, not ideas of any particular thing, but ideas of whole classes of things. If I say "Socrates is mortal," I am thinking of the individual, Socrates. But if I say "Man is mortal," I am thinking, not of any particular man, but of the class of men in general. Such an idea is called a general idea, or a concept. All class-names, such as man, tree, house, river, animal, horse, being, which stand, not for one thing, but for a multitude of things, represent concepts. We form these general ideas by including in them all the qualities which the whole class of objects has in common, and excluding from them all the qualities in which they differ, that is to say, the qualities which some of the objects possess, but others do not. For example, I cannot include the quality whiteness in my general idea of horses, because, though some horses are white, others are not. But I can include the quality vertebrate because all horses agree in being vertebrate. Thus a {144} concept is formed by bringing together the ideas in which all the members of a class of objects agree with one another, and neglecting the ideas in which they differ.