the nature
of the individual sex life.

There are certain modifications of the erotic life which are explicable
only when we recollect that under environmental influences situations
which originally did not call up an emotional response come later to do
so. This fact, which was first noted by Setchenov, was experimentally
demonstrated by Pavlov and his students.[7] They found that when some
irrelevant stimulus, such as a musical tone or a piece of coloured paper
was presented to a dog simultaneously with its food for a sufficiently
long period, the presentation of the tone or paper alone finally caused
the same flow of saliva that the food had originally evoked. The
irrelevant stimulus was named a _food sign_, and the involuntary motor
response of salivary secretion was called a _conditioned reflex_ to
differentiate it from the similar response to the biologically adequate
stimulus of food, which was termed an _unconditioned reflex_.

"The significance of the conditioned reflex is simply this, that an
associated stimulus brings about a reaction; and this associated
stimulus may be from any receptor organ of the body; and it may be
formed of course not merely in the laboratory by specially devised
experiments, but by association in the ordinary environment."[1] Thus it
is evident that the formation of conditioned reflexes takes place in
all fields of animal and human activity.

Watson has recently stated that a similar substitution of one stimulus
for another occurs in the case of an emotional reaction as well as at
the level of the simple physiological reflex response.[8] This means
that when an emotionally exciting object stimulates the subject
simultaneously with one not emotionally exciting, the latter may in time
(or even after one joint stimulation) arouse the same emotional response
as the former. Kempf considers this capacity of the emotion to become
thus conditioned to other than the original stimuli "of the utmost
importance in determining the selections and aversions throughout life,
such as mating, habitat, friends, enemies, vocations, professions,
religious and political preferences, etc."[5]

Just as Pavlov and his followers found that almost anything could become
a food sign, so the study of neurotics has shown that the sexual emotion
can be fixed upon almost any love object. For example, a single
characteristic of a beloved person (e.g.,--eye colour, smile posture,
gestures) can become itself a stimul