His ambitious design is, in his own words, to show "the successive changes in human society, the influence which each instant exerts on the succeeding instant, and thus, in its successive modifications, the advance of the human species towards truth or happiness." Taken literally, this is an impossible design, and to put it forward as a practical proposition is as if a man were to declare his intention of writing a minute diary of the life of Julius Caesar from his birth to his death. By stating his purpose in such terms, Condorcet reveals that he had no notion of the limitations which confine our knowledge of the past, and that even if he had conceived a more modest and practicable programme he would have been incapable of executing it. His formula, however, is worth remembering. For the unattainable ideal which it expresses reminds us how many periods and passages of human experience must always remain books with seven seals. Condorcet distinguished ten periods of civilisation, of which the tenth lies in the future, but he has not justified his divisions and his epochs are not co-ordinate in importance. Yet his arrangement of the map of history is remarkable as an attempt to mark its sections not by great political changes but by important steps in knowledge. The first three periods--the formation of primitive societies, followed by the pastoral age, and the agricultural age--conclude with the invention of alphabetic writing in Greece. The fourth is the history of Greek thought, to the definite division of the sciences in the time of Aristotle. In the fifth knowledge progresses and suffers obscuration under Roman rule, and the sixth is the dark age which continues to the time of the Crusades. The significance of the seventh period is to prepare the human mind for the revolution which would be achieved by the invention of printing, with which the eighth period opens. Some of the best pages of the book develop the vast consequences of this invention. The scientific revolution effected by Descartes begins a new period, which is now closed by the creation of the French Republic.