  Our concept of mass has evolved considerably over the centuries, most notably
from Newton to Einstein, and then even more vigorously with the establishment
of the standard model and the subsequent discovery of the Higgs boson. Mass is
now invoked in various guises depending on the circumstance: it is used to
represent inertia, or as a coupling constant in Newton's law of universal
gravitation, and even as a repository of a mysterious form of energy associated
with a particle at rest. But recent developments in cosmology have demonstrated
that rest-mass energy is most likely the gravitational binding energy of a
particle in causal contact with that portion of the Universe within our
gravitational horizon. In this paper, we examine how all these variations on
the concept of mass are actually interrelated via this new development and the
recognition that the source of gravity in general relativity is ultimately the
total energy in the system.
