The study of the child cannot be accomplished by an "instantaneous" process; his characteristics can only be illustrated cinematographically. "External means," organized in accordance with the needs of psychical life, are of fundamental importance; for how is it possible to judge of individual differences in the acquisition of internal order, in the ascent to abstraction, in the progressive stages of intellectual development, in the achievement of discipline, without the existence of pre-determined and unvarying external means which, like so many points of support, lead the child in process of formation towards his goal? In order to determine _individual differences_ logically, there must be a _constant work or aim_; and this is the external means on which each personality builds itself up. When the external support is the same, and corresponds in general to the psychical needs of a given age, a difference of internal construction is _due to the individual himself_. On the other hand, if the means were different, the variations in reaction might be attributed to differences in the means. Finally, it is obvious that in all scientific research, the _instrument of measurement_ must be fixed. But each _thing to be measured_ requires a special instrument, and the constant instrument in psychical measurement should be "the method of education." A series of formulae, such as the Binet-Simon tests, can neither measure anything, nor give even an approximate idea of intellectual levels of intelligence according to age; as to the children who respond, whence is their response derived? How far is this due to the intrinsic activity of the individual, and how far to the action of environment? And if the portion due to environment be ignored, who can determine what intrinsic psychical value should be given to the response? In each personality we must recognize two parts: one is the individual, natural, spontaneous activity by means of which elements may be taken from the environment wherewith the personality may be elaborated internally, constructed and augmented, and hence _characterized_; another part is the external instrument with which all this may be done. For instance, a child who at the age of four can recognize sixty-four colors, shows that he possesses remarkable activity in the perception of colors, and in the arrangement of them in gradation in his mind, etc.; but he also shows that he has had the means to accomplish this achievement; he has had, for instance, sixty-four color-tablets, with which he has been able to practise at his leisure and undisturbed, as long as was necessary for such assimilation. The psychical factor P is the sum of two factors, one internal, the other external: P = I + E of these the unknown, non-directly measurable factor I may be indicated by X: P = X + E