As we have seen in an earlier chapter, one of the fundamental entities
of the universe is matter. A second, not less important, is called
energy. Energy is indispensable if the world is to continue to exist,
since all phenomena, including life, depend on it. Just as it is humanly
impossible to create or to destroy a particle of matter, so is it
impossible to create or to destroy energy. This statement will be more
readily understood when we have considered what energy is.
Energy, like matter, is indestructible, and just as matter exists in
various forms so does energy. And we may add, just as we are ignorant of
what the negative and positive particles of electricity which constitute
matter really are, so we are ignorant of the true nature of energy. At
the same time, energy is not so completely mysterious as it once was. It
is another of nature's mysteries which the advance of modern science has
in some measure unveiled. It was only during the nineteenth century that
energy came to be known as something as distinct and permanent as matter
itself.