fident that the distinction between images and
sensations is ultimately valid, and I should be glad to be convinced
that images can be reduced to sensations of a peculiar kind. I think it
is clear, however, that, at any rate in the case of auditory and visual
images, they do differ from ordinary auditory and visual sensations, and
therefore form a recognizable class of occurrences, even if it should
prove that they can be regarded as a sub-class of sensations. This is
all that is necessary to validate the use of images to be made in the
sequel.

LECTURE IX. MEMORY

Memory, which we are to consider to-day, introduces us to knowledge in
one of its forms. The analysis of knowledge will occupy us until the end
of the thirteenth lecture, and is the most difficult part of our whole
enterprise.

I do not myself believe that the analysis of knowledge can be effected
entirely by means of purely external observation, such as behaviourists
employ. I shall discuss this question in later lectures. In the present
lecture I shall attempt the analysis of memory-knowledge, both as an
introduction to the problem of knowledge in general, and because memory,
in some form, is presupposed in almost all other knowledge. Sensation,
we decided, is not a form of knowledge. It might, however, have
been expected that we should begin our discussion of knowledge with
PERCEPTION, i.e. with that integral experience of things in the
environment, out of which sensation is extracted by psychological
analysis. What is called perception differs from sensation by the fact
that the sensational ingredients bring up habitual associates--images
and expectations of their usual correlates--all of which are
subjectively indistinguishable from the sensation. The FACT of past
experience is essential in producing this filling-out of sensation, but
not the RECOLLECTION of past experience. The non-sensational elements in
perception can be wholly explained as the result of habit, produced
by frequent correlations. Perception, according to our definition in
Lecture VII, is no more a form of knowledge than sensation is, except
in so far as it involves expectations. The purely psychological problems
which it raises are not very difficult, though they have sometimes been
rendered artificially obscure by unwillingness to admit the fallibility
of the non-sensational elements of perception. On the other hand, memory
raises many difficult and very important problems, w