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When two [[principal]]s cannot affect each other, they are said to
be isolated from one another.  Strictly speaking, if two principals
are isolated from one another, there is no way for one to know
whether the other exists: all interactions with the environment
do not expose proof of the existence or non-existence of the
other.

Typically, principals are isolated from one another with
respect to a particular property.  Common properties include
information-flow (security) and performance.