P = X + E
in which X is the internal and intrinsic part peculiar to the individual life, it may be said that every individual has his X. But in order to approach to direct knowledge of X, it is essential to know P and E.
He who carries out an examination, or supposes himself to be performing a "psychical measurement" by dwelling on psychical results, is in reality measuring a mixture of two unknown quantities, one of which, being external to the individual, nullifies the results of research.
Hence, to study individual differences in isolated activities, such as the perception of colors, musical sounds, the letters of the alphabet; or the capacity for observation of surroundings and the detection of errors; or coordination of movements, language, etc., it is essential to have first determined a constant element: the means of development offered by environment.
Here a simple and clearly defined difference between pedagogy and psychology manifests itself: pedagogy determines experimentally the means of development and the method of applying them while respecting the internal or personal liberty of the individual; psychology studies average reactions or individual reactions in the species or the individual. But the two things are two aspects of a single fact, which is the development of man; the individual and the environment are the two factors X and E of the same product: the psychical entity.
Isolated psychical researches of a moral order must also, if they are to be of any real value, be based upon prolonged observation, after the internal activities have become orderly; because it is easy to make errors of judgment in a chaos. In clinical psychiatry or in criminal pathology, when we speak of "keeping a subject under observation" for purposes of diagnosis, we mean placing him in special surroundings, under hygienic and disciplinary conditions, etc., and observing him for some time in such an environment. Such a process has a value still more extensive and profound in the case of normal individuals in process of evolution. In such a case it is necessary not only to offer orderly external surroundings, but to reduce the chaotic internal world of the child to order, and, after this, to observe him for a considerable time.
We may offer as an illustration the following observations made upon two of the most interesting children who attended our schools. They were admitted into the training school for teachers during my last International Course in Rome.