y and "self-activity" of
the individual, have pushed the stimulus away into the background;
while others, fixing their attention on the stimulus, have treated the
individual as the passive recipient of sensation and "experience"
generally. Experience, however, is not received; it is lived, and that
means done; only, it is done in response to stimuli. The concept of
reaction covers the ground.

While speaking of sensations and thoughts as belonging under the
general head of reactions, it is well, however, to bear in mind that
all mental action tends to arouse and terminate in muscular and
glandular activity. A thought or a feeling tends to "express itself"
in words or (other) deeds. The motor response may be delayed, or
inhibited altogether, but the tendency is always in that direction.

Different Sorts of Stimuli

To call all mental processes reactions means that it is always in
order to ask for the stimulus. Typically, the stimulus is an external
force or motion, such as light or sound, striking on a sense organ.
There are also the internal stimuli, consisting of changes occurring
within the body and acting on the sensory nerves that are distributed
to the muscles, bones, lungs, stomach and most of the organs. The
sensations of muscular strain and fatigue, and of hunger and thirst,
are aroused by internal stimuli, and many reflexes are aroused in the
same way.

Such internal stimuli as these are like the better known external
stimuli in that they act upon sense organs; but it {48} seems
necessary to recognize another sort of stimuli which act directly on
the nerve centers in the brain. These may be called "central stimuli"
and so contrasted with the "peripheral stimuli" that act on any sense
organ, external or internal. To do this is to take considerable
liberty with the plain meaning of "stimulus", and calls for
justification. What is the excuse for thus expanding the notion of a
stimulus?

The excuse is found in the frequent occurrence of mental processes
that are not directly aroused by any peripheral stimulus, though they
are plainly aroused by something else. Anything that arouses a thought
or feeling can properly be called its stimulus. Now it often happens
that a thought is aroused by another, just preceding thought; and it
seems quite in order to call the first thought the stimulus and the
second the response. A thought may arouse an emotion, as when the
thought of my enemy, suddenly occurring to mind