While the terms motive and motivation have crept into psychology from common sense and have never been adequately defined, there is no doubt that they stand for a determinant of behavior which needs physiological interpretation. Before we can attempt this task, it is necessary to clear up certain psychological phases of the problem. The common-sense individual wants to know the underlying purpose of each and every activity. He is interested in the why of behavior, whereas the psychologist is properly interested in the how. Scientific knowledge of any phenomenon develops out of an analysis of its inherent characteristics, not by reading into it some teleological explanation. This is especially true of motivation. The layman asks the question, "Why are you fighting?" and is satisfied by the common-sense answer, "Because he called me a name." The psychologist, however, must face the question of how a certain external stimulus arouses this particular response. He realizes that there is nothing inherent in the nature of this external stimulus which determines the character of the response, and he consequently looks for some intra-organic excitant which must have acted in conjunction with the external stimulus. He seeks, in a word, for the complete stimulus situation, internal as well as external; and for the internal conditions involved in setting up the act of fighting he may, if he chooses, use the term motivational excitant.
There are many internal conditions which help to determine the specificity of response, and the question now arises as to whether all of these may be classed as motivational excitants. In the broadest sense they probably are, the phrase being used interchangeably with set and determination. Psychologists, however, have shown a tendency to class as motivational excitants only those internal conditions which are of biological utility. Thus, the "set to fight" and the "desire to mate" are called motives because they preserve the integrity of the individual and of the race, whereas the "set to add" and the "desire to read" are placed outside the classification. This is teleology again, but in a new dress! Classifications based upon the innateness, the emotional source, or the dependability of the motivational excitant never entirely escape the same difficulty.
Such terms as mating and fighting, which properly characterize overt response, cannot be employed as if they also characterized the internal determinants. If, instead, the actual source is sought in the activity of various bodily tissues, a qualification emerges which is at once apparent and real. In utilizing the fuels supplied to them, certain bodily tissues (such as smooth muscles and glands) give off physicochemical excitants which disrupt the internal equilibrium of the organism until they are relieved. This disruption expresses itself in a more or less random type of behavior, which tends to persist until an external excitant is encountered which will release the mechanism for equilibrating the disturbance. Those internal conditions which can be traced more or less directly to tissue needs of the smooth muscles and glands are called the basic motivational forces; whereas those which can be traced to specific patterns of excitation in the neuromuscular system, and which may have been derived secondarily from more basic tissue conditions, can be referred to by some other term, such as set. Thus the behavior of a hungry animal can be traced directly to a condition in the stomach, while that of a person solving a mathematical problem, if at all related to a stomachic condition, has acquired that relation secondarily as the result of some intervening process. It can be maintained, of course, that every overt action is related to basic tissue conditions in some degree. But the fact that psychologists find it necessary to treat certain outcomes of such conditions as sex, hunger, and thirst in a class by themselves points to a distinctive feature of some kind.