The German Philosopher, Immanuel Kant had some clever things to say (as well as some not so clever things like his distinction between noumena and phenomena and how it relates to free will). One of his cleverer ideas was a kind of moral litmus test he referred to as the "categorical imperative" whose first formulation can be phrased as follows: "Always act according to that maxim whose universality as a law you can at the same time will."
This is clearly connected to the golden rule: "do to others as you would have them do to you" and, in some sense, generalizes the golden rule. It may also remind you of the ad for Dial Soap, which said "Aren't you glad you use Dial? (Don't you wish everybody did?)" If the ad's statement was universally true, it would place using Dial soap under the claims of Kant's categorical imperative.
Of course, what we're talking about here is an exercise of conscience. Not everyone may have the same sense of the value of using Dial Soap (for example). Anyone who has the sense that everybody should use it should (and, presumably, for everyone's good), according to the categorical imperative, use it himself, indeed is morally obliged to do so (presinding, for the moment, from discussing certain highly technical conditions that need not be considered here for this argument).
Kant's idea also connects with the universalizability of human reason. If a rational person could be expected to reach the same conclusions (e.g. about Dial Soap) then the imperative becomes universal and the result is we have identified an absolute moral law.
Unfortunately, this is the weakest link in Kant's argument. The reason is that reason depends upon axioms, and not everyone accepts the same set of axioms. Jean Jacques Rousseau and Aristotle, for example, would be at logger heads because they didn't start with the same set of axioms.
Now as it happens, there are two major areas of application of axioms that philosophers should be concerned with. There are axioms underlying how pure mathematics works, and axioms underlying how realithy itself works. The difference comes from the fact that mathematics is flexible enough to describe alternative realities. The reason that physics is such a challenging science is that mathematical theories only approximate physical reality. No mathematical theory has ever been invented that covers all physical law. On the frontiers of physics multiple mathematical theories are possible, all based upon differrent underlying hypotheses which take the form of axioms. The refinement of axioms is a very subtle art. The axioms developed by Isaac Newton for his theory of gravity are, comparatively speaking, very crude and simplistic compared with the axioms developed by Albert Einstein for his theory of gravity. Some day we will find that Einstein's axioms are relatively crude and simplistic in comparison to what is needed to develop a unified field theory that incorporates gravity as a field phenomenon.
When we compare Rousseau to Aristotle, however, it turns out both philosophers would rightly be considered as having crude axioms, but, to any sane mind, Aristotle's axioms would be considered common sensical and Rousseau's would not. Thus Aristotle's "Nicomachean Ethics" can support the development of a just society (in particular because it finds a place for virtue and reason) and Rousseau's "Du contrat social ou Principes du droit politique" cannot (because it finds no place for either virtue or reason - they are not incorporated into his axiomatic framework). The requirements of Rousseau's "contract" are satisfied by Hitler's Germany in time of war, for example.
One of the claims that make Rousseau's philosophy attractive to the post-modern progressive is that Rousseau suggested his system supported maximum freedom. In reality, however, real freedom is not possible without purposeful self-regulation and virtue. in other words, our contemporary fans of Rousseau are delusional, and, indeed, fundamentally so.
Another of his superficially attractive claims is that the "noble savage" (i.e., one born and living apart from "civilization") is inherently good. In Rousseau's system, he doesn't prove this claim. Rather he takes it as axiomatic. Unfortunately, some savage societies are pretty tame, while others are, well, pretty savage, and, virtually by definition, are incapable of supporting higher forms of human expression and endeavor without undergoing major evolutionary change. So Rousseau is rightly regarded as a philosopher in the Romantic tradition. (Another way to say this is that his head was filled with a lot of romantic twaddle.)
We can summarize the above discussion by saying that Aristotle had, as a result of his upbringing and his long life, a goodly measure of common sense and, by comparison, Rousseau did not. While Rousseau's philosophy did contain some valid arguments and ideas, the structure as a whole was a tissue of holes, while Aristotle's system was comparatively solid and comprehensive. To put this more succinctly, we can say that Aristotle's system comported nicely with reality and Rousseau's didn't.
Finding and critiquing the connection of a given philosophical system with reality can, however, be a highly challenging task. The reason is that one must grasp the axiomatic structure of a system in order to critique it. The vast majority of what passes for philosophical argument these days, however, tends to be highly superficial and fails utterly to recognize the role of axioms in the system in question, often, in part, because the original philosopher did not himself (or herself) recognize their own use of axioms. In other words, they often remained utterly in the dark about their own presuppositions. And it is precisely upon the presuppositions or axioms, and their connection (or lack thereof) with reality that a philosophical system often stands or falls (presuming, of course, that the philosopher, when given a set of axioms, can reason his way out of a paper bag).
In the post-modern era, this disconnection from axioms generally takes on a more sinister shape. Indeed, the typical post-modern tends to be highly suspicious of axioms, and when he or she acknowledges the role of an axiom, their first instinct is to throw it away and to try to rebuild their system on what is left. This task may be summarized by the term "the hermeneutic of suspicion." Unfortunately, even this exercise tends to be conducted in a state of intellectual blindness, so that the exercise of knocking out axioms tends to resemble a game of whackamole - others pop up unobserved in the place of ones that are "whacked." The result is that philosophical papers written by post-moderns tend to have a highly technical patina that covers up for a lack of clear insight or judgment. (Hegel was an early pioneer in this technique, and even Kant often stumbled in this way.)
Perhaps the greatest challenge to the modern philosopher is to articulate a coherent theory of free moral agency, or free will and independent moral responsibility. Such a theory must, of necessity, be axiomatized, depending upon claims that cannot be directly proved on the basis of observation. Without some theory of free moral agency (at least in the sense of a presumption of free will or a set of presumptions that imply free will), no system of ethics can survive. The same applies to the question of human reason. If we cannot presume that the human person possesses a faculty of reason, then no system of ethics can apply. We cannot hold a two year old to the same degree of responsibility of which an adult is generally capable, because their grasp of reality is insufficient.
In the same way, our theory of "what is good" must be axiomatized, not because "one man's meat is another man's poison" but precisely because no theory can be completed without that. Thus, as we saw above, Kant's "categorical imperative" requires supplementary axioms to apply to a specific case. One might say that it is a heuristic, rather than a principle, without additional axioms to anchor it to an application.
It seems to me, however, that it is possible to undertake this task (i.e., the task of discovering axioms that fit reality well and developing sound arguments that build upon them) fruitfully, and the result of doing so may be described as a natural law theory of ethics.
It is high time we did so. Human reason can be productively employed in the task of developing a unified field theory of physics. There is no fundamental reason why human reason cannot be equally productive in the development of a universal theory of moral behavior.
Although we should probably admit that our tendency to sin will retard the project. Just so, civilization may collapse before we can complete our unified field theory or may otherwise prevent us from grasping it firmly.