The study of children is often the only means of testing the truth of our analyses.

If we decide that a certain mental state is due to a union of simpler elements, then we may appeal to the proper period of child life to see the union taking place.

The range of growth is so enormous from the infant to the adult, and the beginnings of the child's mental life are so low in the scale, in the matter of mental endowment, that there is hardly a question of analysis now under debate in psychology which may not be tested by this method.
At this point it is that child psychology is more valuable than the study of the mind of animals.

The latter never become men, while children do.

The animals represent in some few respects a branch of the tree of growth in advance of man, while being in many other respects very far behind him.