The sources of man's behavior (his observable action) and his subjective experience (such as thoughts, feelings, and wishes) are twofold: the external stimuli that impinge on him and the internal dispositions that result from the interaction between inherited physiological characteristics and experience with the world. When we focus on the former, we note that a person acts in such-and-such a way because of certain qualities in a situation. For example, he attacks a friend because the friend insulted him, or he loses interest in a lecture because the teacher is dull or uninformed, or he fails in his program of study because the necessity of supporting himself through school leaves insufficient time for studying. It is evident that a man's behavior varies greatly from moment to moment, from circumstance to circumstance, changing with the changing conditions to which he is exposed.
Still, even as we recognize the dependency of behavior on outside stimuli, we are also aware that it cannot be accounted for on the basis of the external situation alone, but that in fact it must arise partly from personal characteristics. For example, the same quantity of alcohol that will induce loss of control and dilapidated behavior in one individual will produce scarcely noticeable effects in another. Some of this variation, of course, can be laid to momentary physical and social conditions. For instance, when the social circumstances of drinking are friendly and benign, a person may be less guarded and more inclined to permit himself to get drunk than when the situation is hostile or dangerous. Moreover, the amount and type of food in the stomach before and during drinking also determine the alcoholic effect by influencing the rate of absorption of the alcohol into the bloodstream and its subsequent effects on the brain.
But some of the variations in the effects of alcohol or any other stimulus result from stable personal characteristics that are almost always in operation in all situations. Body weight, for example, is quite influential in determining the physiological and psychological effects given quantities of alcohol can produce. Then too, psychologically, some persons are more concerned than others with exercising controls or restraints over behavior, so they keep the lid on the dilapidating and disinhibiting effects of alcohol, sometimes even until they lose consciousness or go to sleep. But other personality characteristics can lead a person to lose control very rapidly and grow either depressed, unruly, and hostile, or exceedingly sociable and outgoing. Or, to take a nonalcoholic example, one person may virtually never get discouraged and give up trying to accomplish something even when the circumstances warrant it, while another quits trying at the first evidence of trouble.
It is certain that a person's behavior is governed not only by momentary external stimulation, but also by the stable attributes he carries about with him. Clearly, then, we must identify these attributes, or dispositions, if we are to understand and predict psychological reactions. For these attributes are what we mean by personality. And the identifiable reactions are the end result of the interplay between them and immediate situations. In short, both external stimuli and personality must be taken into account in understanding and predicting human behavior and subjective experience.