The present age is not merely an epoch of discovery; it is also a period of revision of the various elements of knowledge. Having recognised that there are no phenomena of which the first cause is still accessible, science has resumed the examination of her ancient certitudes, and has proved their fragility. To-day she sees her ancient principles vanishing one by one. Mechanics is losing its axioms, and matter, formerly the eternal substratum of the worlds, becomes a simple aggregate of ephemeral forces in transitory condensation. Despite its conjectural side, by virtue of which it to some extent escapes the severest form of criticism, history has not been free from this universal revision. There is no longer a single one of its phases of which we can say that it is certainly known. What appeared to be definitely acquired is now once more put in question.