  It has recently been argued that the inability to measure the absolute phase
of an electromagnetic field prohibits the representation of a laser's output as
a quantum optical coherent state. This argument has generally been considered
technically correct but conceptually disturbing. Indeed, it would seem to place
in question the very concept of the coherent state. Here we show that this
argument fails to take into account a fundamental principle that not only
re-admits the coherent state as legitimate, but formalizes a fundamental
concept about model building in general, and in quantum mechanics in
particular.
