This insistence involves the substitution of a doctrine of mental characters for the popular conception of vague and indefinite traits and peculiarities. How is this doctrine to be derived? Obviously from psychology itself. Neither biometry nor biology nor common sense can furnish the materials.
Look with me for a moment, if you will, to see what psychology has to offer. It is evident that the general psychology of the average normal mind will not suffice when the matter is one of defining differences among minds of the same class. Just as physical inheritance must take account of arrays and schemes of distribution, and not of averages, so must mental qualities and magnitudes be arranged with respect to definite individual variations within the class.
A psychology of individual differences is thus invoked; and a psychology of individual differences does exist; or rather, it is in process. The way of scientific description is first to reveal uniformities hidden in the mass, and afterward to seek the rule of variation from the average. General psychology, taken in this sense, is accordingly the older branch of the science—the psychology of what is common to all minds—individual psychology, the newer. So it happens that although the older branch of the science has a well-developed metrical technique, established at almost the same moment that "The Origin of Species" appeared, its quantitative determinations are determinations of psychophysical constants and not laws of individual variation. These laws can not then be used (at least not directly) for the statistical study of inheritance. What is needed is a psychology of typical differences, and this it is that individual psychology is by way of supplying. Let me, in a word, indicate its method. It proceeds by experiment to take the dimensions of mind as regards variable functions, e. g., the maximal amount read in a given time and under given conditions, or the number of words remembered or of figures added. The first results show typical differences as between mind and mind. The experimenter next proceeds to factor the performance into elementary processes and functions. By drawing his conditions closer and closer he discovers that the capacity for reading depends upon such simple factors as the range of consciousness, the degree of attention, and the temporal rate of visual processes, factors all capable of measurement and exact description. He discovers that remembrance depends upon the employment of visual or auditory or kinesthetic processes, i. e., that in one observer an eye-mind, in another an ear-mind and in a third a muscle-mind is employed. Of these factors, he can predicate heritability. It is as if "criminality" were reduced to a lack of motor control plus an abnormally intensive passion or lust. "Criminality" would then never be inherited, but the constituent factors might very well be. The method of individual psychology, however, goes farther. After
