When one is convinced that our knowledge of the outer world is limited to sensations, we can no longer understand how it is possible to give oneself up, as physicists do, to speculations upon the constitution of matter.
Up to the present there have been three principal ways of explaining the physical phenomena of the universe.

The first, the most abstract, and the furthest from reality, is above all verbal.

It consists in the use of formulas in which the quality of the phenomena is replaced by their magnitude, in which this magnitude, ascertained by the most precise processes of measurement, becomes the object of abstract reasoning which allows its modifications to be foreseen under given experimental conditions.

This is pure mathematics, a formal science depending upon logic.

Another conception, less restricted than the above, and of fairly recent date, consists in treating all manifestations of nature as forms of energy.