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.