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

If we were to compare two children, one of whom has had at his
disposal the sixty-four colors in the conditions described above, and
another who has been left to himself in poor surroundings, where gray
and brown tints prevail, and who seems dull and unobservant, etc., we
should find a very remarkable psychical difference. Such a difference
is not, however, intrinsic; it might well be that, subjected to the
same conditions as the first child, the second would recognize the
sixty-four colors. The judgment we should give in such a case would be
based upon an external factor, not upon internal potentialities. We
should really be appraising two different environments, not two
different individuals.

To enable us to judge of individual differences, it would be necessary
for the two children to have had _the same means of development_. In
this case, if at the same age they were not equally capable of
distinguishing the sixty-four colors, but if, for instance, one of the
two could recognize only thirty of these, a true individual psychical
difference would be apparent. One of the tests proposed by one of the
greatest authorities on experimental psychology in Italy, to determine
the intellectual level of sub-normal (backward or deficient)
children, was to make a child pick out the largest and the smallest
cube in a series. This choice, in common with nearly all the tests
proposed for the same purpose, we considered quite independently of
the influence of _culture_ and _education_; and it was appreciated as
the expression of an intimate, personal activity of the intelligence
itself. But if one of the deficient children I had educated on my
method had been subjected to the test, he would, in virtue of a long
sensory training, have chosen the largest and the smallest cube very
much more easily than the chi