e is material for reflection here.

Nevertheless the more we reflect on that material, the more evident
does it become that the plain man cannot be wrong in believing in the
external world which seems revealed in his experiences. We find that
all attempts to discredit it rest upon the implicit assumption of its
existence, and fall to the ground when that existence is honestly
denied. So our problem changes its form. We no longer ask: Is there
an external world? but rather: _What_ is the external world, and how
does it differ from the world of mere ideas?

[1] "The Grammar of Science," 2d Ed., London, 1900, pp. 60-63.

CHAPTER IV

SENSATIONS AND "THINGS"

15. SENSE AND IMAGINATION.--Every one distinguishes between things
perceived and things only imagined. With open eyes I see the desk
before me; with eyes closed, I can imagine it. I lay my hand on it and
feel it; I can, without laying my hand on it, imagine that I feel it.
I raise my eyes, and see the pictures on the wall opposite me; I can
sit here and call before my mind the image of the door by which the
house is entered.

What is the difference between sense and imagination? It must be a
difference of which we are all somehow conscious, for we unhesitatingly
distinguish between the things we perceive and the things we merely
imagine.

It is well to remember at the outset that the two classes of
experiences are not wholly different. The blue color that I imagine
seems blue. It does not lose this quality because it is only
imaginary. The horse that I imagine seems to have four legs, like a
horse perceived. As I call it before my mind, it seems as large as the
real horse. Neither the color, nor the size, nor the distribution of
parts, nor any other attribute of the sort appears to be different in
the imaginary object from what it is in the object as given in
sensation.

The two experiences are, nevertheless, not the same; and every one
knows that they are not the same. One difference that roughly marks
out the two classes of experiences from one another is that, as a rule,
our sense-experiences are more vivid than are the images that exist in
the imagination.

I say, as a rule, for we cannot always remark this difference.
Sensations may be very clear and unmistakable, but they may also be
very faint and indefinite. When a man lays his hand firmly on my
shoulder, I may be in little doubt whether I feel a sensation or do
not; but when h