  The concept of the physical state of a system is ubiquitous in physics but is
usually presented in terms of specific cases. For example, the state of a point
particle of mass m is completely characterized by its position and momentum.
There is a tendency to consider such states as "real", i.e., as physical
properties of a system. This rarely causes problems in classical physics but
the notion of real quantum states has contributed mightily to the philosophical
conundrums associated with quantum mechanics. The Einstein-Rosen-Podolsky
paradox is a prime example. In fact, quantum states are not physical properties
of a system but rather subjective descriptions that depend on the information
available to a particular observer. This realization goes a long way toward
resolving such dilemmas as Schr\"odinger's cat, wave function collapse, quantum
non-locality, and parallel universes.
