  The natural unit system, in which the value of fundamental constants such as
c and h are set equal to one and all quantities are expressed in terms of a
single unit, is usually introduced as a calculational convenience. However, we
demonstrate that this system of natural units has a physical justification as
well. We discuss and review the natural units, including definitions for each
of the seven base units in the International System of Units (SI) in terms of a
single unit. We also review the fundamental constants, which can be classified
as units-dependent or units-independent. Units-independent constants, whose
values are not determined by human conventions of units, may be interpreted as
inherent constants of nature.
