The human mind, it must be apparent to every reader, is not less complex than the automobile. On the contrary, it is infinitely more complex and subject to an infinitely wider range of variations. As has been pointed out above, it is not necessary for practical, every-day purposes to measure every possible variation and every one of the infinite number of dimensions of any human mind in order to ascertain the individual’s ability to succeed in the ordinary pursuits of life. But even in our ordinary, every-day affairs and contacts, in the simplest forms of employment, there are called into play such a number of different sorts of ability and mental power that there must be applied, if one is really to know of what a particular individual is capable, a large variety of tests of different kinds for measuring different powers. And for the mental measurement of individuals whose work calls for the highest development and capacity, a still larger variety of tests must be applied. It is not always possible—in fact, it is extremely difficult—to devise tests that do not to some degree measure the mental content resulting from education and experience, in the effort to measure the mental capacity which limits and controls one’s education and experience. The qualities that determine capacity are inherent in the individual. One is born with them or is not born with them. In their whole infinite variety they are not all possessed by any one individual, and the particular grouping of mental qualities which any one person inherits is probably not possessed by any other person living or who has ever lived. Yet while individuals differ so completely that it can truthfully be said that Nature never cast two persons in the same mold, yet there are qualities possessed by all intelligent persons, the simpler and more elemental expressions of which are absolutely essential to intelligent life and existence, and these can be so grouped, classified, measured, and standardized as to provide a scale whereby the inherent capacity with respect to these important and essential qualities may be determined equally in the case of the totally illiterate, untrained labourer or artisan and the highly trained, educated product of a university postgraduate course.