Hence, to establish systematic tests for a certain purpose, such as that of preparing children to write, without taking their ages into account, is valueless. For example, my system of writing is based upon the direct preparation of the movements which physiologically concur to produce writing: _i.e._ manipulation of the instrument of writing and the tracing of the letters of the alphabet. The children, filling in the contours of the insets with innumerable parallel strokes in the one case, and touching the sand-paper letters in the other, fix the two muscular mechanisms so perfectly, that the final result is an "explosion" of "spontaneous writing" extraordinarily uniform in all the children--because, as if all molded to a common form, they have fixed the necessary movements by touching the same alphabet, and therefore reproduce its forms faithfully. To bring this about, to establish a real motor-mechanism, it is essential that the exercise should be repeated over and over again. Now the children who take most interest in filling in the figures with parallel strokes, and, above all, in touching the letters, are, at most, between four and five years old. If we offer the same material to a child of six he will not touch the letters often enough, and he will always write imperfectly, in comparison with the child who has begun the exercise at a suitable age. This applies also to all the other details of the system. It is therefore possible to determine experimentally, with, I believe, a precision not hitherto attained, what is the mental attitude of the child at various ages, and hence, if the fitting material for development be offered, what will be the average level of intellectual development according to age. Here we have an indication of the possibility of _determining_ the means of development so exactly as to establish a true correspondence between internal needs and external stimuli, just as actual as the correspondence which exists between the insect and the flower. He who has all this material ready to his hand has an easy task in bringing about the natural development of the psychic life of the child. With such objects at his disposal, every teacher may realize the ideal of _liberty in the school_. This long, occult experiment--suggested to me, as I have already said, by Itard and Seguin--is, in fact, my initial contribution to education. All this preparatory work has served for the determination of the method now well known, but it is also the key to its continuation. * * * * *