ortant point concerning images, which will
occupy us much hereafter, and that is, their resemblance to previous
sensations. They are said to be "copies" of sensations, always as
regards the simple qualities that enter into them, though not always
as regards the manner in which these are put together. It is generally
believed that we cannot imagine a shade of colour that we have never
seen, or a sound that we have never heard. On this subject Hume is the
classic. He says, in the definitions already quoted:

"Those perceptions, which enter with most force and violence, we may
name IMPRESSIONS; and under this name I comprehend all our sensations,
passions and emotions, as they make their first appearance in the soul.
By IDEAS I mean the faint images of these in thinking and reasoning."

He next explains the difference between simple and complex ideas, and
explains that a complex idea may occur without any similar complex
impression. But as regards simple ideas, he states that "every simple
idea has a simple impression, which resembles it, and every simple
impression a correspondent idea." He goes on to enunciate the general
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" ("Treatise of Human Nature," Part I,
Section I).

It is this fact, that images resemble antecedent sensations, which
enables us to call them images "of" this or that. For the understanding
of memory, and of knowledge generally, the recognizable resemblance of
images and sensations is of fundamental importance.

There are difficulties in establishing Hume's principles, and doubts
as to whether it is exactly true. Indeed, he himself signalized an
exception immediately after stating his maxim. Nevertheless, it is
impossible to doubt that in the main simple images are copies of similar
simple sensations which have occurred earlier, and that the same is true
of complex images in all cases of memory as opposed to mere imagination.
Our power of acting with reference to what is sensibly absent is largely
due to this characteristic of images, although, as education advances,
images tend to be more and more replaced by words. We shall have much
to say in the next two lectures on the subject of images as copies of
sensations. What has been said now is merely by way of reminder that
this is their most notable characteristic.

I am by no means con