Upon a basis of interior order produced by internal organization, the mind then builds up its castle with the same leisurely calm with which a living organism grows spontaneously after birth.
We can give but a primary idea at present of the practical possibility of determining average levels of interior development according to age. We shall further require many perfect experiments, in which homogeneous children, completely suitable environment, and trained teachers will afford adequate material for observation. Then students will be able to undertake a scientific work, which will perhaps be characterized by a precision superior even to that with which it is at present possible to measure the body, and give the mathematical averages of growth.
We must consider, however, that the indications available to-day represent a long, systematic toil, and that they rest upon the still greater labor of finding external material means for natural development.
This will give some idea of the difficulty of scientific researches, which many still believe it possible to make by means of arbitrary and superficial tests such as those of Binet and Simon!
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."