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. [Footnote 1: "Measuring Intelligence," _Harper's Magazine_, March, 1920.] There have been a number of tests devised for the purpose of estimating an individual's general intelligence.[1] On a rating scale such as is used in these examinations most individuals will come up to a certain standard that may be called average or normal. There will be a certain number so far below the normal rating in a complex of traits that go to produce intelligent (competent and facile) behavior that they will have to be classed as subnormal, ranging from feeblemindedness to idiocy. A certain number will be found so extraordinarily gifted in general traits and in specific abilities--in given subject-matters, as, for example, in mathematics and music--that they will be marked out as geniuses. Following the laws of probability, the greater the inferiority or superiority, the more exceptional it will be.