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.

Now reason is the faculty of concepts. This may not, at first sight,
be obvious. Reason, it might be objected, is the faculty of arguing,
of drawing conclusions from premises. But a little consideration will
show us that, though this is so, yet all reasoning is employed upon
concepts. All reasoning is either deductive or inductive. Induction
consists in the formulation of general principles from particular
cases. A general principle is always a statement made, not about a
particular thing, but about a whole class of things, that is, about a
concept. Concepts are formed inductively by comparing numerous
examples of a class. Deductive reasoning is always the opposite
process of applying general principles to particular cases. If we
argue that Socrates must be mortal because all men are so, the
question is whether Socrates is a man, that is to say, whether the
concept, man, is properly applied to the particular object called
Socrates. Thus inductive reasoning is concerned with the formation of
concepts, deductive reasoning with the application of them.

Socrates, in placing all knowledge in concepts, was thus making reason
the organ of knowledge. This was in direct opposition to the principle
of the Sophists, who placed all knowledge in sense-perception. Now
since reason is the universal element in man, it follows that
Socrates, in identifying knowledge with concepts, was restoring the
belief in an objective truth, valid for all men, and binding