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.