Quote Originally Posted by English assassin
It seems to me prison should be for punishment for the offender, for deterance for everyone else, and for rehabilitation of the offender.
I agree with you in principle, but have some conceptual problems with that. Deterance and rehabilitation are clear but what exactly is punishment and what is it for? I reject the notion of some metaphysical "guilt" that has to be redeemed by inflicting aversive situations on humans. In a progressive society punishment has to be justified in a social-engineering view.
Theoretically, punishment by prison serves four purposes:

General prevention - people are deterred from breaking the law by the fact that they would receive punishment.
Individual prevention - by punishing anti-social behavior a pro-social learning process should be induced in the offender
Exclusion - while locked up, offenders can't harm society any further
Order of Law - the authority of the law must be integrated part of society, so that it is part of culture that acts against the law have repercussion