mind is suggested by our analysis of consciousness, images
are what would naturally suggest themselves. But since we found
that images can only be defined causally, we cannot deal with this
suggestion, except in connection with the difference between physical
and psychological causal laws.

I come next to those characteristics of mental phenomena which arise out
of mnemic causation. The possibility of action with reference to what
is not sensibly present is one of the things that might be held to
characterize mind. Let us take first a very elementary example. Suppose
you are in a familiar room at night, and suddenly the light goes out.
You will be able to find your way to the door without much difficulty
by means of the picture of the room which you have in your mind. In this
case visual images serve, somewhat imperfectly it is true, the purpose
which visual sensations would otherwise serve. The stimulus to the
production of visual images is the desire to get out of the room, which,
according to what we found in Lecture III, consists essentially of
present sensations and motor impulses caused by them. Again, words heard
or read enable you to act with reference to the matters about which they
give information; here, again, a present sensible stimulus, in virtue of
habits formed in the past, enables you to act in a manner appropriate
to an object which is not sensibly present. The whole essence of the
practical efficiency of "thought" consists in sensitiveness to signs:
the sensible presence of A, which is a sign of the present or future
existence of B, enables us to act in a manner appropriate to B. Of
this, words are the supreme example, since their effects as signs are
prodigious, while their intrinsic interest as sensible occurrences on
their own account is usually very slight. The operation of signs may or
may not be accompanied by consciousness. If a sensible stimulus A calls
up an image of B, and we then act with reference to B, we have what may
be called consciousness of B. But habit may enable us to act in a manner
appropriate to B as soon as A appears, without ever having an image of
B. In that case, although A operates as a sign, it operates without the
help of consciousness. Broadly speaking, a very familiar sign tends to
operate directly in this manner, and the intervention of consciousness
marks an imperfectly established habit.

The power of acquiring experience, which characterizes men and animals,
is an