  Effective classicality of a property of a quantum system can be defined using
redundancy of its record in the environment. This allows quantum physics to
approximate the situation encountered in the classical world: The information
about a classical system can exist independently from its state. In quantum
theory this is no longer possible: In an isolated quantum system the state and
the information about it are inextricably linked, and any measurement may --
and usually will -- reset that state. However, when the information about the
state of a quantum system is spread throughout the environment, it can be
treated (almost) as in classical physics.
