a real essence, which will consist of their physical constitution, but this is in the main unknown
to us, and is not the "essence" of which scholastics speak. Essence, as we can know it, is purely
verbal; it consists merely in the definition of a general term. To argue, for instance, as to
whether the essence of body is only extension, or is extension plus solidity, is to argue about
words: we may define the word "body" either way, and no harm can result so long as we adhere
to our definition. Distinct species are not a fact of nature, but of language; they are "distinct
complex ideas with distinct names annexed to them." There are, it is true, differing things in
nature, but the differences proceed by continuous gradations: "the boundaries of the species,
whereby men sort them, are made by men." He proceeds to give instances of monstrosities,
concerning which it was doubtful whether they were men or not. This point of view was not
generally accepted until Darwin persuaded men to adopt the theory of evolution by gradual
changes. Only those who have allowed themselves to be afflicted by the scholastics will realize
how much metaphysical lumber it sweeps away.
Empiricism and idealism alike are faced with a problem to which, so far, philosophy has found
no satisfactory solution. This is the problem of showing how we have knowledge of other
things than ourself and the operations of our own mind. Locke considers this problem, but what
he says is very obviously unsatisfactory. In one place * we are told: "Since the mind, in all its
thoughts and reasonings, hath no other immediate object but its own ideas, which it alone does
or can contemplate, it is evident that our knowledge is only conversant about them." And again:
"Knowledge is the perception of the agreement or disagreement of two ideas." From this it
would seem to follow immediately that we cannot know of the existence of other people, or of
the physical world, for these, if they exist, are not merely ideas in any mind. Each one of us,
accordingly, must, so far as knowledge is concerned, be shut up in himself, and cut off from all
contact with the outer world.
This, however, is a paradox, and Locke will have nothing to do with paradoxes. Accordingly, in
another chapter, he sets forth a different theory, quite inconsistent with the earlier one. We
have, he
* Op. cit., Book IV, Ch. I.