The concept of self is, however, a common-sense concept; it has, as we have seen, its practical side; and you will understand, therefore, that _the differential study of selves has a high practical importance_. Such a study is not rigorously or exclusively psychological. But since certain ‘mental traits,’ and certain combinations of them, may render a man fit or unfit for a proposed business or profession, it is important to know in what degree these traits are present; and here the psychologist is of assistance; he has helped to devise ‘mental tests’ which serve to identify and measure them. It is also especially important to know what traits are likely to be found together, and in what degree. This problem has been vigorously attacked, of recent years, on the side of intellect; and while the details belong to a chapter in practical psychology (p. 33) which we cannot here open, there is one result, at any rate, which should find a place in a scientific text-book. There seems to be no doubt that _the individual nervous system possesses, over and above its special habits, susceptibilities, tendencies, and activities, a characteristic manner of functioning at large_; so that a =common or general factor= enters into all the special intellectual responses that are called forth by particular situations. It is not easy to make this result clear to the reader, mainly because no one has as yet a clear idea of what the common or general factor is; we have good evidence that it exists, but we can say very little more about it. Different names have been given to it: ‘energy of attention,’ ‘general ability,’ ‘intellective energy,’ ‘general intelligence’; but they indicate the way in which it manifests itself, and not its own nature; the best name for the present is the vague ‘general common factor.’ We do not know, either, upon what it depends: on blood-supply, perhaps, or on the arrangement of nervous structures, or on some individual ‘quality’ of the nervous elements, or perhaps on something else that we cannot even guess at. What it does is to hold a man’s intellectual traits together and to enter into the exhibition of them all; _it is thus, from the psychological point of view, a sort of supreme determining tendency, guiding all mental processes whatsoever into the channels of intellectual selfhood_. Whether there is a like general factor on the emotive side, and whether ‘emotive energy’ is of the same kind as this ‘intellective energy,’ cannot be said.