cannot but
believe that any disagreement on this matter only arises from some want of
mutual understanding; and, therefore, in order to anticipate any criticisms
to which the present work may be open on this score, I append this
explanatory note.

I readily grant that the term "persistence of force" is not a happy one,
seeing that the word "force," as used by physicists, does not at the
present time convey the full meaning which Mr. Spencer desires it to
convey. But I think that any impartial physicist will be prepared to admit
that, in the present state of his science, we are entitled to conclude that
energy of position is merely the result of energy of motion; or, in other
words, that potential energy is merely an expression of the fact that the
universe, as a whole, is replete with actual energy, whose essential
characteristic is that it is indestructible. And this may be concluded
without committing ourselves to any particular theory as to the physical
explanation of gravity; all we need assert is, that in some way or other
gravity is the result of ubiquitous energy. And this, it seems to me, we
must assert, or else conclude that gravity can never admit of a physical
explanation. For all that we mean by a physical explanation is the proved
establishment of an equation between two quantities of energy; so that if
energy of position does not admit of being interpreted in terms of energy
of motion, we must conclude that it does not admit of being interpreted at
all--at least not in any physical sense.

Throughout the foregoing essays, therefore, I have assumed that all forms
of energy are but relatively varying expressions of the same fact--the
fact, namely, which Mr. Spencer means to express when he says that force is
persistent. And it seems to me almost needless to show that this fact is
really the basis of all science. For unless this fact is assumed as a
postulate, not only would scientific inquiry become impossible, but all
experience would become chaotic. The physicist could not prosecute his
researches unless he presupposed that the forces which he measures are of a
permanent nature, any more than could the chemist prosecute his researches
unless he presupposed that the materials which he estimates by energy-units
are likewise of a permanent nature. And similarly with all the other
sciences, as well as with every judgment in our daily experience. If,
therefore, any one should be hypercritical enough to dispute