s governed by habit and past experience. If you listen to a man
playing the pianola without looking at him, you will have images of his
hands on the keys as if he were playing the piano; if you suddenly look
at him while you are absorbed in the music, you will experience a shock
of surprise when you notice that his hands are not touching the notes.
Your image of his hands is due to the many times that you have heard
similar sounds and at the same time seen the player's hands on the
piano. When habit and past experience play this part, we are in the
region of mnemic as opposed to ordinary physical causation. And I think
that, if we could regard as ultimately valid the difference between
physical and mnemic causation, we could distinguish images from
sensations as having mnemic causes, though they may also have physical
causes. Sensations, on the other hand, will only have physical causes.

However this may be, the practically effective distinction between
sensations and images is that in the causation of sensations, but not
of images, the stimulation of nerves carrying an effect into the brain,
usually from the surface of the body, plays an essential part. And
this accounts for the fact that images and sensations cannot always be
distinguished by their intrinsic nature.

Images also differ from sensations as regards their effects. Sensations,
as a rule, have both physical and mental effects. As you watch the train
you meant to catch leaving the station, there are both the successive
positions of the train (physical effects) and the successive waves
of fury and disappointment (mental effects). Images, on the contrary,
though they MAY produce bodily movements, do so according to mnemic
laws, not according to the laws of physics. All their effects, of
whatever nature, follow mnemic laws. But this difference is less
suitable for definition than the difference as to causes.

Professor Watson, as a logical carrying-out of his behaviourist theory,
denies altogether that there are any observable phenomena such as
images are supposed to be. He replaces them all by faint sensations, and
especially by pronunciation of words sotto voce. When we "think" of a
table (say), as opposed to seeing it, what happens, according to him, is
usually that we are making small movements of the throat and tongue
such as would lead to our uttering the word "table" if they were more
pronounced. I shall consider his view again in connection with words