Neuromuscular action is in a perpetual state of flux, and repetition of identical stimuli may not involve the same or even similar responses. Behavior changes, not only from infancy to old age, but also from year to year, from day to day, from hour to hour, and from moment to moment. From the standpoint of the student who seeks an analogy in the physical sciences, the task of analyzing such a situation must seem rather hopeless. It is only when we realize that the difference between the variability of physical and biological phenomena is mainly one of degree that we gain the proper perspective. Indeed, it now seems that the most significant facts of physics are facts of relative change and motion, Be that as it may, physiological psychology long ago had to, shift from a study of the organism as a relatively stable aggregate of reflexes, instincts, and habits to an investigation of the conditions under which reaction changes occur.
The more accurately observations are made, the more conspicuous. human variability becomes. With sufficiently fine units of measurement, no two instances of human reactions have ever been proved to beidentical, precise records showing an indefinable number of combinations of latency, amplitude, acceleration, retardation, and recovery. This holds for such relatively simple neuromuscular events as the knee-jerk. and such well practiced behavior as is represented by the writing of one's signature. The scientific question is not the existence of variability, but how much, under what conditions, and with what consequences.
In many respects this represents our most important as well as our most difficult problem: important, because in it belong the physiological explanations of practically all the major topics which have interested psychologists in the last few decades--motivation, set and attitude, learning, fatigue and oscillation, intelligence, and temperament; difficult, because the neural dynamics underlying these conditions are little understood.
Psychology is filled with terms expressive of alleged relationships between behavior and the neuromuscular mechanism--determining tendencies, conditioning, bonds, satisfyingness, drive, and libido. Practically none of these terms is grounded on well-recognized physiological principles. For the most part, their usage offers an excellent example of what Holt has so aptly called "verbal magic . . . faith in the reifying power of mere words" and the assumption that giving a name to a phenomenon suffices to explain it.
Actually, however, such verbiage as organic need, instinctive drive, etc., obscures the real subtleties of the problem to both the writer and the reader. Grounded as he is in both physiology and neurology, the physiological psychologist must either put specific meaning into these terms or discard them altogether from his thinking. There are, of course, good concepts as well as bad concepts, working hypotheses as well as sterile hypotheses; and a great part of our task will be the separation and reclamation of those hypotheses which rest upon well-known principles of neural functioning, and which suggest possible extensions therefrom. These theories, when treated, should be taken rather lightly by the student. Some of them are probably true only in part; many others may be shown by later work to be entirely wrong. Their function is mainly that of bridging the gaps in our present knowledge; and if they serve to point out new lines of experimental attack which will narrow the gap, they will have served a useful end. The fact that, as a result of newly acquired knowledge, the bridge no longer carries any weight is a purely incidental occurrence.