While discussing morality on Uncommon Descent it became the point of one commenter that morality only needs the Golden Rule to ground and make morals objective and imperative. I replied that it was hard to see how this could be the case, especially since not even the Golden Rule was, per se, a moral imperative itself. I spoke of the morality of "fairness", and afterward thought that wasn't the best way to put things. Further thought revealed to me that, though it wasn't the best way to put things, it wasn't particularly bad, either.
The question to be resolved was whether, minus a transcendent source, i.e. God, to ground morals and make them actually binding --- to give them real oughtness --- morality was anything more than a system that is really governed by man and his whims, making them utterly arbitrary. If the latter was the case, then, I contend morality reduces obviously to, "Man governs morality, and morality doesn't govern men."
Now, when we talk of grounding morality, we are really asking for the source of morals. But we need to be very careful, because to one that may be saying something more than to another. Firstly, there needs to be agreement that morals are actually binding at all; are we really, cosmically, required to keep some set of rules? Most people simply intuitively and reflexively mean just that when they say "morality". Morality, to them, can be nothing other than that which is required of man, full stop. If someone denies that morality is objectively about what men ought to practice, then discussion needs to focus elsewhere; at that point, the source is irrelevant. If, however, there is agreement that men are really required to keep these moral rules, then, one may proceed to an argument for the necessity of those morals being grounded in something which can give meaning to any idea of an actual oughtness, or requirement in keeping, said rules. If we agree that men really ought to behave morally, then where could possibly such a binding requirement have come from?
Hoping that the preceding has set the table for the main points to follow, let me examine the point of said commenter above, that the Golden Rule is the source that gives weight to the oughtness of morality.
The Golden Rule, that we should do unto others as we would have them do unto us, can be viewed in two ways. Neither of those ways, however, addresses the question being asked.
In one way, the Golden Rule is simply a maxim that tells us how to apply the rules. However, what it doesn't tell us, is whether there are any rules that need to be applied; only that, IF there are any rules that ACTUALLY need to be applied, this is an easy way of knowing HOW to apply them. This way of seeing the Golden Rule, therefore, simply does not speak to the existence of moral laws, which clearly means it is not a source for the moral laws either.
In the second way, however, you might say that the Golden Rule itself is one of the rules that needs to be applied; it is, in itself, one of the moral rules that we ought to follow (fairness, not putting one's self first). However, if this is the case, then it doesn't speak to the question of the source of the moral law at all, but is simply an instance of what the moral law might say. It, in fact, would then be one of the rules in need of explanation itself.
I think both ways of thinking of the Golden Rule are correct. It is both one of the laws itself, and a fitting maxim that tells us how to apply the moral law(s). What it clearly isn't, though, is a fitting candidate as the source, the ground, of morality.
The reason the Golden Rule may be mistaken as a fitting source of the moral law is this, that in one sense, it may actually get near to the source of man knowing the moral law, not only as a maxim to help apply it, but also in how one may naturally come to realize that other human beings should be regarded as worthy as one's self as deserving to being treated well and equitably (i.e., fairness). But do you see that? To explain how the Golden Rule gets close to being a proposed source of the moral law, I had to invoke some yet more base moral laws to uphold it; namely that of treating others well and equitably. If the Golden Rule itself must rely on other base moral laws even to make it appear as a serious candidate for the source of the moral law, clearly it fails. The Golden Rule, then, may be likened to a teacher that "learned us our 'rithmetic". The teacher clearly isn't the source of arithmetic, only how we came to know, understand, and apply the arithmetical rules.
Additionally, this is a type of genetic fallacy, perhaps not easy to see. In this case, the fallacy is that if we explain how people came to know or understand something that would itself add an objective factual foundation that the belief was true (without questioning why or how it could be true), while clearly it is possible to have confidence in how we came to know something and have it be false after all, or perhaps true but not fully in accord with how or why we thought it to be true.
For these reasons, the Golden Rule doesn't seem to be a valid candidate for the grounding of morality, that which actually gives rise to oughtness.
If man is the source of the moral law, then man governs the moral law, and the moral law does not govern man.
If God is the source of the moral law, then the moral law is objectively required to be adhered to by man, exactly in accord with how we perceive it to actually be.