energies composing one and the same system, a mutual dependence which is
just what has made the extension of the principle possible by
measurements suitably chosen. If, therefore, the philosopher applies
this principle to the solar system complete, he must at least soften its
outlines. The law of the conservation of energy cannot here express the
objective permanence of a certain quantity of a certain thing, but
rather the necessity for every change that is brought about to be
counterbalanced in some way by a change in an opposite direction. That
is to say, even if it governs the whole of our solar system, the law of
the conservation of energy is concerned with the relationship of a
fragment of this world to another fragment rather than with the nature
of the whole.

It is otherwise with the second principle of thermodynamics. The law of
the degradation of energy does not bear essentially on magnitudes. No
doubt the first idea of it arose, in the thought of Carnot, out of
certain quantitative considerations on the yield of thermic machines.
Unquestionably, too, the terms in which Clausius generalized it were
mathematical, and a calculable magnitude, "entropy," was, in fact, the
final conception to which he was led. Such precision is necessary for
practical applications. But the law might have been vaguely conceived,
and, if absolutely necessary, it might have been roughly formulated,
even though no one had ever thought of measuring the different energies
of the physical world, even though the concept of energy had not been
created. Essentially, it expresses the fact that all physical changes
have a tendency to be degraded into heat, and that heat tends to be
distributed among bodies in a uniform manner. In this less precise form,
it becomes independent of any convention; it is the most metaphysical of
the laws of physics since it points out without interposed symbols,
without artificial devices of measurements, the direction in which the
world is going. It tells us that changes that are visible and
heterogeneous will be more and more diluted into changes that are
invisible and homogeneous, and that the instability to which we owe the
richness and variety of the changes taking place in our solar system
will gradually give way to the relative stability of elementary
vibrations continually and perpetually repeated. Just so with a man who
keeps up his strength as he grows old, but spends it less and less in
actions, and c