have a sensation, it is true, but you also know the table is hard. How
do you know it? I cannot tell you; you simply know it, and cannot help
knowing it; and that is the end of the matter.

Reid's doctrine was not without its effect upon other philosophers.
Among them we must place Sir William Hamilton (1788-1856), whose
writings had no little influence upon British philosophy in the last
half of the last century.

Hamilton complained that Reid did not succeed in being a very good
_Natural Realist_, and that he slipped unconsciously into the position
he was concerned to condemn. Sir William tried to eliminate this
error, but the careful reader of his works will find to his amusement
that this learned author gets his feet upon the same slippery descent.
And much the same thing may be said of the doctrine of Herbert Spencer
(1820-1903), who claims that, when we have a sensation, we know
directly that there is an external thing, and then manages to sublimate
that external thing into an Unknowable, which we not only do not know
directly, but even do not know at all.

All of these men were anxious to avoid what they regarded as the perils
of Idealism, and yet they seem quite unable to retain a foothold upon
the position which they consider the safer one.

Reid called his doctrine the philosophy of "Common Sense," and he
thought he was coming back from the subtleties of the metaphysicians to
the standpoint of the plain man. That he should fall into difficulties
and inconsistencies is by no means surprising. As we have seen
(section 12), the thought of the plain man is far from clear. He
certainly believes that we perceive an external world of things, and
the inconsistent way in which Descartes and Locke appeal from ideas to
the things themselves does not strike him as unnatural. Why should not
a man test his ideas by turning to things and comparing the former with
the latter? On the other hand, he knows that to perceive things we
must have sense organs and sensations, and he cannot quarrel with the
psychologists for saying that we know things only in so far as they are
revealed to us through our sensations. How does he reconcile these two
positions? He does not reconcile them. He accepts them as they stand.

Reid and various other philosophers have tried to come back to "Common
Sense" and to stay there. Now, it is a good position to come back to
for the purpose of starting out again. The experience of the pl