The life of a man is a double series--a series of effects produced in him by the rest of the world, and a series of effects produced in that world by him. A man's make-up or nature equals his tendencies to be influenced in certain ways by the world and to react in certain ways to it. If we could thus adequately describe each of a million human beings--if, for each one, we could prophesy just what the response would be to every possible situation of life--the million men would be found to differ widely. Probably no two out of the million would be so alike in mental nature as to be indistinguishable by one who knew their entire natures. Each has an individuality which marks him off from other men. We may study a human being in respect to his common humanity, or in respect to his individuality. In other words, we may study the features of intellect and character which are common to all men, to man as a species; or we may study the differences in intellect and character which distinguish individual men. Individuals are commonly considered as differing in respect to such traits either quantitatively or qualitatively, either in degree or in kind. A quantitative difference exists when the individuals have different amounts of the same trait. Thus, "John is more attentive to his teacher than James is"; "Mary loves dolls less than Lucy does"; "A had greater devotion to his country than B had"; are reports of quantitative differences, of differences in the amount of what is assumed to be the same kind of thing. A qualitative difference exists when some quality or trait possessed by one individual is lacking in the other. Thus, "Tom knows German, Dick does not"; "A is artistic, B is scientific"; "C is a man of thought, D is a man of action"; are reports of the fact that Tom has some positive amount or degree of the trait "knowledge of German" while Dick has none of it; that A has some positive amount of ability and interest in art while B has zero; whereas B has a positive amount of ability in science, of which A has none; and so on. A qualitative difference in intellect or character is thus really a quantitative difference wherein one term is zero, or a compound of two or more quantitative differences. All intelligible differences are ultimately quantitative. The difference between any two individuals, if describable at all, is described by comparing the amounts which A possesses of various traits with the amounts which B possesses of the same traits. In intellect and character, differences of kind between one individual and another turn out to be definable, if defined at all, as compound differences of degree.