possible but in particular
instances they may very nearly approach to each other. Thus in sleep, in
a fever, in madness, or in any very violent emotions of soul, our ideas
may approach to our impressions; as, on the other hand, it sometimes
happens, that our impressions are so faint and low that we cannot
distinguish them from our ideas. But notwithstanding this near
resemblance in a few instances, they are in general so very different,
that no one can make a scruple to rank them under distinct heads, and
assign to each a peculiar name to mark the difference" ("Treatise of
Human Nature," Part I, Section I).

I think Hume is right in holding that they should be ranked under
distinct heads, with a peculiar name for each. But by his own confession
in the above passage, his criterion for distinguishing them is not
always adequate. A definition is not sound if it only applies in cases
where the difference is glaring: the essential purpose of a definition
is to provide a mark which is applicable even in marginal cases--except,
of course, when we are dealing with a conception, like, e.g. baldness,
which is one of degree and has no sharp boundaries. But so far we have
seen no reason to think that the difference between sensations and
images is only one of degree.

Professor Stout, in his "Manual of Psychology," after discussing various
ways of distinguishing sensations and images, arrives at a view which is
a modification of Hume's. He says (I quote from the second edition):

"Our conclusion is that at bottom the distinction between image
and percept, as respectively faint and vivid states, is based on a
difference of quality. The percept has an aggressiveness which does not
belong to the image. It strikes the mind with varying degrees of force
or liveliness according to the varying intensity of the stimulus. This
degree of force or liveliness is part of what we ordinarily mean by
the intensity of a sensation. But this constituent of the intensity of
sensations is absent in mental imagery"(p. 419).

This view allows for the fact that sensations may reach any degree of
faintness--e.g. in the case of a just visible star or a just audible
sound--without becoming images, and that therefore mere faintness cannot
be the characteristic mark of images. After explaining the sudden shock
of a flash of lightning or a steam-whistle, Stout says that "no mere
image ever does strike the mind in this manner"(p. 417). But I believe
that thi