It has been remarked recently that physicists are now divided into two
opposing schools according to the way in which they view the subject of
energy, some regarding it as a mere mathematical abstraction and others
looking upon it as a physical entity, filling space and continuously
migrating by definite paths from one place to another. It may be added
that there are numerous factions within these two parties; for instance,
not all of those who consider energy to be something more than a mere
mathematical expression would maintain that a given quantity of it retains
its identity just as a given quantity of matter does. In fact a close
analysis would possibly show that opinions are graded very closely and
continuously from a view hardly differing from that of Lagrange, who
clearly saw and freely used the mathematical considerations involving
energy before the word had been invented or its physical meaning
developed, up to that stated recently in its extreme form by Professor
Ostwald, who would replace what he terms a mechanical theory of the
universe by an "energetical" theory, and would dwell exclusively on energy
as opposed to its vehicles.

Differences of opinion of this sort very frequently reduce to differences
of definition, and in this case the meaning of the word "identity" or some
similar word or phrase has undoubtedly much to do with the view that is
taken of the matter. It may be interesting, for instance, to look for a
moment at our ideas of the identity of matter and the extent to which they
are influenced by the accepted theory of its constitution.

Very few persons would hesitate to admit that the matter that now
constitutes the universe is identical in amount with that which
constituted it one million years ago, and that any given portion of that
matter is identical with an equal amount of matter that then existed,