hich it is necessary
to consider at the first possible moment.

One reason for treating memory at this early stage is that it seems to
be involved in the fact that images are recognized as "copies" of
past sensible experience. In the preceding lecture I alluded to Hume's
principle "that all our simple ideas in their first appearance are
derived from simple impressions, which are correspondent to them, and
which they exactly represent." Whether or not this principle is liable
to exceptions, everyone would agree that is has a broad measure of
truth, though the word "exactly" might seem an overstatement, and
it might seem more correct to say that ideas APPROXIMATELY represent
impressions. Such modifications of Hume's principle, however, do not
affect the problem which I wish to present for your consideration,
namely: Why do we believe that images are, sometimes or always,
approximately or exactly, copies of sensations? What sort of evidence is
there? And what sort of evidence is logically possible? The difficulty
of this question arises through the fact that the sensation which an
image is supposed to copy is in the past when the image exists, and can
therefore only be known by memory, while, on the other hand, memory of
past sensations seems only possible by means of present images. How,
then, are we to find any way of comparing the present image and the past
sensation? The problem is just as acute if we say that images differ
from their prototypes as if we say that they resemble them; it is the
very possibility of comparison that is hard to understand.* We think
we can know that they are alike or different, but we cannot bring them
together in one experience and compare them. To deal with this problem,
we must have a theory of memory. In this way the whole status of images
as "copies" is bound up with the analysis of memory.

* How, for example, can we obtain such knowledge as the
following: "If we look at, say, a red nose and perceive it,
and after a little while ekphore, its memory-image, we note
immediately how unlike, in its likeness, this memory-image
is to the original perception" (A. Wohlgemuth, "On the
Feelings and their Neural Correlate with an Examination of
the Nature of Pain," "Journal of Psychology," vol. viii,
part iv, June, 1917).

In investigating memory-beliefs, there are certain points which must
be borne in mind. In the first place, everything constituting a