we
must speak.

Why should we speak of it? The universe is an assemblage of solar
systems which we have every reason to believe analogous to our own. No
doubt they are not absolutely independent of one another. Our sun
radiates heat and light beyond the farthest planet, and, on the other
hand, our entire solar system is moving in a definite direction as if it
were drawn. There is, then, a bond between the worlds. But this bond may
be regarded as infinitely loose in comparison with the mutual dependence
which unites the parts of the same world among themselves; so that it is
not artificially, for reasons of mere convenience, that we isolate our
solar system: nature itself invites us to isolate it. As living beings,
we depend on the planet on which we are, and on the sun that provides
for it, but on nothing else. As thinking beings, we may apply the laws
of our physics to our own world, and extend them to each of the worlds
taken separately; but nothing tells us that they apply to the entire
universe, nor even that such an affirmation has any meaning; for the
universe is not made, but is being made continually. It is growing,
perhaps indefinitely, by the addition of new worlds.

Let us extend, then, to the whole of our solar system the two most
general laws of our science, the principle of conservation of energy and
that of its degradation--limiting them, however, to this relatively
closed system and to other systems relatively closed. Let us see what
will follow. We must remark, first of all, that these two principles
have not the same metaphysical scope. The first is a quantitative law,
and consequently relative, in part, to our methods of measurement. It
says that, in a system presumed to be closed, the total energy, that is
to say the sum of its kinetic and potential energy, remains constant.
Now, if there were only kinetic energy in the world, or even if there
were, besides kinetic energy, only one single kind of potential energy,
but no more, the artifice of measurement would not make the law
artificial. The law of the conservation of energy would express indeed
that _something_ is preserved in constant quantity. But there are, in
fact, energies of various kinds,[88] and the measurement of each of them
has evidently been so chosen as to justify the principle of conservation
of energy. Convention, therefore, plays a large part in this principle,
although there is undoubtedly, between the variations of the different