We should now be in a position to begin a direct attack upon our problem: the development of a clear and comprehensive theory of motivation. In order that the theory shall be comprehensive it must take cognizance of most of the concepts and principles which have been considered in our review of previous discussions. In order, however, to be clear it must avoid or improve upon the methods of thinking which have been used in some of these earlier expositions. In the first place, we must shun what may be called the fallacy of simplicity, under the influence of which we may try to show that some single factor in human life is actually responsible for the entire system. Secondly, we must avoid both the confusion of physiological with psychological concepts, and the rejection of either of these as unimportant. Perrin and Klein, in beginning their discussion of "the motivation of behavior" say: "The student will find it profitable at this point not to attempt to distinguish too closely between physiological and psychological phenomena." Without denying that this may be good advice to a beginner, it is the least profitable of all possible admonitions for the purposes which we now have in mind, namely, that of arriving at as complete an understanding as possible of the entire group of facts which relate to human or animal action.
We can approach the problem of motivation from either the psychological or the physiological side. Two distinct systems of facts have to be considered, each requiring methods which are appropriate to it alone; and finally the results of these two treatments must be coördinated into a psychophysical doctrine. Unfortunately, we are afflicted with approximately equivalent amounts of ignorance with respect to each side of the question. Certain conceptions and principles are very clearly established regarding the physiological mechanisms involved in behavior, but--sad to say--our information becomes almost nil when we consider the more complex nervous processes which must constitute the key to voluntary human reactions. It is just on this level of the motivational scheme that the introspective data of psychology, together with introspectively founded concepts and principles, supply the greatest amount of information. When it comes to the simpler processes, again, introspection becomes woefully weak. Thus in a certain sense, the physiological and psychological ideas are complementary to each other; but we should be very, very careful not to let this mislead us into supposing that we can combine them directly into a single system.
All things considered, it seems most advisable to make the first attack upon the problem of motivation from the physiological angle; but in discussing the general principles of the neuromuscular or response mechanism, we shall do well to indicate the relation in which consciousness, or the psychical system stands to the physiological one.