What they are concerned with is things and their changes, and they do not consider such matters as these as falling within their province.

If a botanist could not distinguish between the changes which take place in a plant, and the changes which take place in his sensations as he is occupied in studying the plant, but should tell us that the plant grows smaller as one recedes from it, we should set him down as weak-minded.
That the distinction is everywhere drawn, and that we must not obliterate it, is very evident.

But we are in the presence of what has seemed to many men a grave difficulty.

Are not things presented in our experience only as we have sensations?
what is it to perceive a thing?
is it not to have sensations?
how, then, _can_ we distinguish between sensations and things?
We certainly do so all the time, in spite of the protest of the philosopher; but many of us do so with a haunting sense that our behavior can scarcely be justified by the reason.
Our difficulty, however, springs out of an error of our own.

Grasping imperfectly the full significance of the word "sensation," we extend its use beyond what is legitimate, and we call by that name experiences which are not sensations at all.