The term personality describes the sum total of an individual organism's reactions

The term personality describes the sum total of an individual organism's reactions as seen in relation to those of other organisms of the same species. The concept of the norm or average reaction is now so well grounded in psychological theory as to be taken for granted. Not so apparent, however, are the major axes along which organisms differentiate themselves. From what has been said earlier concerning the intimate relation of heredity and environment, it is obvious that differentiating factors cannot be conceived in terms of constitutional disposition or experience alone. Constitutional potentialities, carried in the "protoplasmic substrate," determine the thresholds for response and limit the range of stimuli to which the organism reacts. On the other hand, the variety of stimuli available helps determine the degree and direction in which hereditary potentialities are exercised. As physiological psychologists, we are especially interested in the hereditary potentialities; but we may not neglect the fact that these dispositions, whatever their nature, are influenced both positively and negatively by the environment in which they develop.

Each reaction is in a sense peculiar to the organism that makes it, and each reaction leaves behind it a trace of its peculiar quality. As these traces combine and recombine to influence subsequent response, the picture of the individual personality assumes an almost unfathomable complexity. It has been observed, however, that the individual qualities of reaction tend to fall into constellations, often called "personality traits." Thus we find qualities of reaction revealing extroversion, sociability, aggressiveness, alertness, etc. The list of these terms could be continued almost indefinitely; but since they are descriptive rather than definitive, we do not know which of them deal with fundamental modes of variation and which with non-essentials. At present it seems best to take a very broad view of the problem, and to distinguish only the intellectual and the non-intellectual aspects of personality. Variations in these two general modes of response-variability, together with their permutations and combinations, might conceivably account for the majority of so-called personality traits.

The essential worth of the dichotomy suggested above, or any other classification of personality traits, rests ultimately upon structuro-functional relationships. A great deal of attention has consequently been given to the correlation of psychologically discernable traits with variations in body structure and function. Psychological tests are far from infallible; but by correlating them with morphological and physiological data, we may be able to use one set of factors more or less as criteria of the other. If some correlations are high or consistent, they offer valuable evidence of a unifying principle common to the two types of measures; if no correlations are significant, however, they suggest either that our a priori dichotomy does not represent a natural organization of the psychophysiological factors which determine personality differences, or that our psychological tests and physiological measures are not sufficiently subtle.