The fact that individuals do differ, and in specific and definable respects, has most serious consequences for social life. It means, briefly, that while general inferences may be drawn from wide and accurate observations of the workings of human nature, these inferences remain general and tentative, and if taken as rigid rules are sure to be misleading. Theories of education and social reform certainly gain from the general laws that can be formulated about original human traits, fatigue, memory, learning capacity, and the like. But they must, if they are to be applicable, take account also, in a precise and systematic way, of the variety of men's interests and capacities. To this fact of variety in the original nature of different men social institutions and educational methods must be adapted. Arbitrary rules that apply to human nature in general do not apply to the specific cases and specific types of talent and desires. Educational and social organizations can mould these, but the result of these environmental influences will vary with individual differences in original capacities. We can waste an enormous amount of time and energy trying to train a person without mechanical or mathematical gifts to be an engineer. We not only save energy and time, but promote happiness, if we can train individuals so that their specific gifts will be capitalized at one hundred per cent. They will be at once more useful to society and more content with themselves, when they are using to the full their own capacities. They will at once be unproductive and unhappy when they find themselves in activities or social situations where their genuine talents are given no opportunity and where their defects put them at a conspicuous handicap.
Individuals differ, it must further be noted, not only in specific traits, but in that complex of traits which is commonly called "intelligence." In the broadest terms, we mean by an individual's intelligence his competence and facility in dealing with his environment, physical, social, and intellectual. This competence and facility, in so far as it is a native endowment, consists of a number of traits present in a more or less high degree, traits, for example, such as curiosity, flexibility of native and acquired reactions, sociability, sympathy, and the like. In a sense an individual possesses not a single intelligence, but many, as many as there are types of activity in which he engages. But one may classify intelligence under three heads, as does Thorndike:[1] mechanical intelligence, involved in dealing with things; social intelligence, involved in dealing with other persons; and abstract intelligence, involved in dealing with the relations between ideas. Each of these types of intelligence involves the presence in a high degree of a group of different traits. Thus, in social intelligence, a high degree of sympathy, sensitivity to praise and blame, leadership, and the like, are more requisite than they are for intelligent behavior in the realm of mechanical operations or of mathematical theory. A person may be highly intelligent in one of these three spheres and mentally helpless in the others. Thus, a brilliant philosopher may be nonplused by a stalled motor; a successful executive may be a babe in the realm of abstract ideas. But what we rate as a person's general intelligence is a kind of average struck between his various competences, an estimate of his general ability to control himself in the miscellaneous variety of situations of which his experience consists.