I have just posted at http://www.erights.org/elib/capability/3parts.html an excellent essay by Bill Frantz, which originally appeared on the SPKI list and has since been reposted by Jonathan on one of the EROS lists, where I saw it. With Bill's permission, I've now published it as a web page. (The title is also mine, but Bill, feel free to tell me to change it.)
This is perhaps the clearest statement I've seen about the dangers of
implicitly confusing a person with the programs that supposedly execute on
their behalf -- the implicit assumption behind "Principle"-based
architectures. Also relevant is
http://www.mediacity.com/~norm/CapTheory/Principal.html by Norm.
Enjoy,
--MarkM