This formula, being only a summary of experience, should be justified and explained by the latter. For this purpose we can borrow facts from two distinct sources: (a) individual development, which is the safest, clearest, and easiest to observe; (b) the development of the species, or historical development, according to the accepted principle that phylogenesis and ontogenesis follow the same general line. I _First Period._ We are already acquainted with it: it is the imaginative age. In normal man, it begins at about the age of three, and embraces infancy, adolescence, youth: sometimes a longer, sometimes a shorter period. Play, romantic invention, mythic and fantastic conceptions of the world sum it up first; after that, in most, imagination is dependent on the influence of the passions, and especially sexual love. For a long time it remains without any rational element. Nevertheless, little by little, the latter wins a place. Reflection--including under the term the working of the intelligence--begins very late, grows slowly, and the proportion as it asserts itself, gains an influence over the imaginative activity and tends to reduce it. This growing antagonism is represented in the following figure. The curve IM is that of the imagination during this first period. It rises at first very slowly, then attains a rapid ascent and keeps at a height that marks its greatest attainment in this earliest form. The dotted line RX represents the rational development that begins later, advances much more slowly, but progressively, and reaches at X the level of the imaginative curve. The two intellectual forms are present like two rivals. The position MX on the ordinate marks the beginning of the second period. [Illustration] _Second Period._ This is a critical period of indeterminate length, in any case, always much briefer than the other two. This critical moment can be characterized only by its causes and results. Its causes are, in the physiological sphere, the formation of an organism and a fully developed brain; in the psychologic order, the antagonism between the pure subjectivity of the imagination and the objectivity of ratiocinative processes; in other words, between mental instability and stability. As for the results, they appear only in the third period, the resultant of this obscure, metamorphic stage. _Third Period._ It is definite: in some way or another and in some degree the imagination has become rationalized, but this change is not reducible to a single formula.