[[license text="""
Copyright © 2007 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.2 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts.  A copy of the license is included
in the section entitled [[GNU_Free_Documentation_License|/fdl.txt]].
"""]]

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.