we can find an infinity of others which
likewise account for all the peculiarities revealed by experiment.
But, as a matter of fact, no one has ever succeeded in giving an
indisputable mechanical representation of the whole physical world.
Even were we disposed to admit the strangest solutions of the problem;
to consent, for example, to be satisfied with the hidden systems
devised by Helmholtz, whereby we ought to divide variable things into
two classes, some accessible, and the others now and for ever unknown,
we should never manage to construct an edifice to contain all the
known facts. Even the very comprehensive mechanics of a Hertz fails
where the classical mechanics has not succeeded.

Deeming this check irremediable, many contemporary physicists give up
attempts which they look upon as condemned beforehand, and adopt, to
guide them in their researches, a method which at first sight appears
much more modest, and also much more sure. They make up their minds
not to see at once to the bottom of things; they no longer seek to
suddenly strip the last veils from nature, and to divine her supreme
secrets; but they work prudently and advance but slowly, while on the
ground thus conquered foot by foot they endeavour to establish
themselves firmly. They study the various magnitudes directly
accessible to their observation without busying themselves as to their
essence. They measure quantities of heat and of temperature,
differences of potential, currents, and magnetic fields; and then,
varying the conditions, apply the rules of experimental method, and
discover between these magnitudes mutual relations, while they thus
succeed in enunciating laws which translate and sum up their labours.

These empirical laws, however, themselves bring about by induction the
promulgation of more general laws, which are termed principles. These
principles are originally only the results of experiments, and
experiment allows them besides to be checked, and their more or less
high degree of generality to be verified. When they have been thus
definitely established, they may serve as fresh starting-points, and,
by deduction, lead to very varied discoveries.

The principles which govern physical science are few in number, and
their very general form gives them a philosophical appearance, while
we cannot long resist the temptation of regarding them as metaphysical
dogmas. It thus happens that the least bold physicists, those who have
wanted