My understanding, rb2k_, is that if I make an offer for compensation in return for a service, and you then go perform that service, then we've made a contract. I "signed" my name by making the offer. You "signed" your name by accepting and completing the service. We've both acted out of free will and we've had a "meeting of the minds." If I back out and try to avoid paying you, then I have broken this contract.

Think how many things in our society would break down if this wasn't the case.