The golden rule says we should do unto others what we would have them do unto us. The problem, however, is the statement is almost entirely vacuous in a pluralistic (and especially relativistic) society. Some people would prefer some things done to them that others would not. For example, some people would rather be treated politely, even if it means telling them lies – perhaps that 2 + 2 = 5. Others would prefer hearing truth, however inconvenient.
Interestingly, but not surprisingly, we find iterations of the golden rule in all major religious traditions: Christianity, Islam, Judaism, Hinduism, etc. I’ll refrain from quotations; but trust me, they’re there.
My point is, as profound as the golden rule initially seems, it comes up rather empty in terms of moral content. By itself, it doesn’t solve many of the more important moral questions, so either we must supplement the golden rule, or consider it about as helpful as Kant’s principle of universalizability, or a crosswalk button.
Here is a better starting moral axiom, I think: Love your neighbor as yourself, where “to love” should be interpreted as “will the good of the other, as other.” Now we’re getting somewhere, but more questions remain; specifically, what is really good for someone? Are some things even really good, at all, or are things only merely apparently good, instrumentally good, etc? If the answer is affirmative of the latter position — that there are no real, objective goods — then all moral systems encounter a two-stage collapse: first, into relativism, and then, into nihilism. But both nihilism and relativism are false, so the former must be affirmed. Some things – at least some things – are really, truly, and objectively good. So now the mission is to figure out just what those sorts of things are.
Traditional natural law, which arises in part from Aristotelian virtue ethics, provides the best account. We have an objective nature (or essence), according to which certain actions either contribute to, or frustrate, the flourishing of what we are, which is rational animality. Truth, for example, would be one such basic good for us, because, as creatures with intellect, we are inclined (by our very nature) toward understanding. Truth, then, truly increases us, causes us to flourish. And because we all want what is really good for us, we should all then pursue truth, even if it means encountering inconvenience. We should not accept being lied to, even if it (at present) makes us more comfortable.
The moral project, then, at least from a TNL perspective, becomes one of better understanding human nature and the actions/behaviors/habits/materials that perfect it. Again, traditionally, the virtues have been identified as both means and end to living a naturally good life: prudence, temperance, fortitude, justice. Whereas vices (vainglory, lust, glutton, avarice, and so on) ultimately signal a privation – of something lacking in us (better moral reasoning, for example) that is otherwise due – a defect, to put it mildly. When we are vicious, we are less than what we could, and, in fact, should, be.
We can now supplement the golden rule as follows: do unto others what you would have them do unto you (assuming, of course, you want others to promote what is really good for you!)