music that we know, we can go
through it in our mind from beginning to end without any discoverable
tendency to suppose that we are really hearing it. But although such
cases are so clear that no confusion seems possible, there are many
others that are far more difficult, and the definition of images is by
no means an easy problem.

To begin with: we do not always know whether what we are experiencing is
a sensation or an image. The things we see in dreams when our eyes are
shut must count as images, yet while we are dreaming they seem like
sensations. Hallucinations often begin as persistent images, and only
gradually acquire that influence over belief that makes the patient
regard them as sensations. When we are listening for a faint sound--the
striking of a distant clock, or a horse's hoofs on the road--we think
we hear it many times before we really do, because expectation brings
us the image, and we mistake it for sensation. The distinction between
images and sensations is, therefore, by no means always obvious to
inspection.*

* On the distinction between images and sensation, cf.
Semon, "Die mnemischen Empfindungen," pp. 19-20.

We may consider three different ways in which it has been sought to
distinguish images from sensations, namely:

(1) By the less degree of vividness in images;

(2) By our absence of belief in their "physical reality";

(3) By the fact that their causes and effects are different from those
of sensations.

I believe the third of these to be the only universally applicable
criterion. The other two are applicable in very many cases, but
cannot be used for purposes of definition because they are liable to
exceptions. Nevertheless, they both deserve to be carefully considered.

(1) Hume, who gives the names "impressions" and "ideas" to what may,
for present purposes, be identified with our "sensations" and "images,"
speaks of impressions as "those perceptions which enter with most force
and violence" while he defines ideas as "the faint images of these (i.e.
of impressions) in thinking and reasoning." His immediately following
observations, however, show the inadequacy of his criteria of "force"
and "faintness." He says:

"I believe it will not be very necessary to employ many words in
explaining this distinction. Every one of himself will readily perceive
the difference betwixt feeling and thinking. The common degrees of these
are easily distinguished, though it is not im