stimuli are not single, that responses are not all alike, and
that learning is not all done under the same drive. Many
stimuli are present when a response is conditioned, all be-
coming discriminative and secondarily reinforcing; many
responses are capable of obtaining the same reinforcement;
and more than one drive (hunger, thirst, etc.) may be satisfied
at various times by the same responses and in the presence of
the same stimuli. Truly, the study of behavior is a lofty chal-
lenge to scientific imagination and method!

There is no book on secondary reinforcement to which we can send
you for additional information. Hull, however, in his Principles of
behavior ( 1943), has devoted a chapter to this important principle.
(Almost any text in social or abnormal psychology will, of course,
provide you with numerous examples of its operation.) You ought now,
however, to be able to follow with little trouble the growing experi-
mental literature in this area. Access to these reports may be gained
by way of the Psychological Abstracts, a journal that contains short
summaries of all articles and books assumed to have any interest what-
ever for psychologists.

In terms of the relation of behavior to the environment of an organ-
ism, the present chapter takes us almost as far as we can go. No more
than two functions of stimuli remain to be considered, and nothing
new will be said about the basic processes of conditioning, extinction,
and the like. In fact, if you have followed our discussion up to this point
with moderate success, you possess most of the available tools for
behavioral analysis. From now on, we shall be interested in the kind
of environmental control that induces changes in the state of an or-
ganism-as when we deprive an animal of food or water. In our talk
of motivation and emotion in the next two chapters, you will see that
we do little more than shift the focus of our attention -- say, from
reinforcement to some previously unmentioned conditions under which
stimuli become reinforcing. In Chapter 11, we shall try to point out
the direction one might profitably take in carrying the principles into
the realm of human interaction.

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