s criterion fails in very much the same instances as those in
which Hume's criterion fails in its original form. Macbeth speaks of--

that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs
Against the use of nature.

The whistle of a steam-engine could hardly have a stronger effect than
this. A very intense emotion will often bring with it--especially
where some future action or some undecided issue is involved--powerful
compelling images which may determine the whole course of life, sweeping
aside all contrary solicitations to the will by their capacity for
exclusively possessing the mind. And in all cases where images,
originally recognized as such, gradually pass into hallucinations, there
must be just that "force or liveliness" which is supposed to be always
absent from images. The cases of dreams and fever-delirium are as
hard to adjust to Professor Stout's modified criterion as to Hume's. I
conclude therefore that the test of liveliness, however applicable in
ordinary instances, cannot be used to define the differences between
sensations and images.

(2) We might attempt to distinguish images from sensations by our
absence of belief in the "physical reality" of images. When we are aware
that what we are experiencing is an image, we do not give it the kind of
belief that we should give to a sensation: we do not think that it has
the same power of producing knowledge of the "external world." Images
are "imaginary"; in SOME sense they are "unreal." But this difference
is hard to analyse or state correctly. What we call the "unreality" of
images requires interpretation it cannot mean what would be expressed
by saying "there's no such thing." Images are just as truly part of the
actual world as sensations are. All that we really mean by calling an
image "unreal" is that it does not have the concomitants which it would
have if it were a sensation. When we call up a visual image of a chair,
we do not attempt to sit in it, because we know that, like Macbeth's
dagger, it is not "sensible to feeling as to sight"--i.e. it does not
have the correlations with tactile sensations which it would have if it
were a visual sensation and not merely a visual image. But this means
that the so-called "unreality" of images consists merely in their not
obeying the laws of physics, and thus brings us back to the causal
distinction between images and sensations.