A license is free if:
  • You can freely use it.
  • You can freely modify it.
  • You can freely share it, with or without modification, with others.

This works for permissive licenses, the official position is in 4 points: http://www.gnu.org/philosophy/free-sw.html

See also "Maintaining Permissive-Licensed Files in a GPL-Licensed Project: Guidelines for Developers" at http://www.softwarefreedom.org/resources/2007/gpl-non-gpl-collaboration.html