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 he touches my back very lightly, I may easily be in
doubt, and may ask myself in perplexity whether I have really been
touched or whether I have merely imagined it. As a vessel recedes and
becomes a mere speck upon the horizon, I may well wonder, before I feel
sure that it is really quite out of sight, whether I still see the dim
little point, or whether I merely imagine that I see it.