If we study as a whole the phenomenon of the development of the human mind, whether by the rational method or by the empirical method, we discover beneath all the apparent irregularities a fundamental law to which its course is necessarily and invariably subject. This law consists in the proposition that man's intellectual system, considered in all its parts, has necessarily assumed in turn three distinct characters: a theological character, a metaphysical character, and lastly a positive or physical character. Thus man began by conceiving phenomena of all kinds as due to the direct and continuous influence of supernatural agents; he next considered them as produced by different abstract forces residing in matter, but distinct and heterogeneous; finally, he limited himself to considering them as subject to a certain number of invariable natural laws, which are nothing other than the general expression of relations observed in their development.

All those who have sufficient understanding of the state of the human mind in the different ages of civilization can easily verify the truth of this general fact. One very simple observation can guide us towards this confirmation, now that this revolution has been carried out for the major part of our ideas. The education of the individual, insofar as it is spontaneous, necessarily displays the same principal phases as does that of the species, and vice versa. And today any man in tune with his times will readily certify of himself that he has been by nature a theologian in his childhood, a metaphysician in his youth, and a physicist in his manhood.