tells us, three kinds of knowledge of real existence. Our knowledge of our own existence is
intuitive, our knowledge of God's existence is demonstrative, and our knowledge of things present
to sense is sensitive (Book IV, Ch. III).
In the next chapter, he becomes more or less aware of the inconsistency. He suggests that some
one might say: "If knowledge consists in agreement of ideas, the enthusiast and the sober man are
on a level." He replies: "Not so where ideas agree with things." He proceeds to argue that all
simple ideas must agree with things, since "the mind, as has been showed, can by no means make
to itself" any simple ideas, these being all "the product of things operating on the mind in a natural
way." And as regards complex ideas of substances, "all our complex ideas of them must be such,
and such only, as are made up of such simple ones as have been discovered to coexist in nature."
Again, we can have no knowledge except (1) by intuition, (2) by reason, examining the agreement
or disagreement of two ideas, (3) "by sensation, perceiving the existence of particular things"
(Book IV, Ch. III, Sec. 2).
In all this, Locke assumes it known that certain mental occurrences, which he calls sensations,
have causes outside themselves, and that these causes, at least to some extent and in certain
respects, resemble the sensations which are their effects. But how, consistently with the principles
of empiricism, is this to be known? We experience the sensations, but not their causes; our
experience will be exactly the same if our sensations arise spontaneously. The belief that
sensations have causes, and still more the belief that they resemble their causes, is one which, if
maintained, must be maintained on grounds wholly independent of experience. The view that
"knowledge is the perception of the agreement or disagreement of two ideas" is the one that Locke
is entitled to, and his escape from the paradoxes that it entails is effected by means of an
inconsistency so gross that only his resolute adherence to common sense could have made him
blind to it.
This difficulty has troubled empiricism down to the present day. Hume got rid of it by dropping
the assumption that sensations have external causes, but even he retained this assumption
whenever he forgot his own principles, which was very often. His fundamental maxim, "no idea
without an antecedent impression," which he takes over from Locke, is only plausible so long as
we think of impressions