  If moral judgement is a rational enterprise, it must be subject to constraints of consistency. What is demanded of us when we are required to be consistent in our moral judgements? The answer is that we are required to apply our principles consistently, that is, to apply the same principle to similar cases. It is inconsistent to apply the principle ‘don’t lie’ to cases involving one’s friends and not to those that involve strangers. If you want to behave in that sort of way, your principle is going to have to be ‘don’t lie to your friends’. What this tells us, of course, is that consistency is not the only requirement.  Our moral principles are supposed to be impartial, and it is not obvious that the principle ‘don’t lie to your friends’meets this condition. But at least someone who takes it as his principle can tell the truth to his friends and lie to strangers without inconsistency.