We humans are all subject to universal natural standards.
Our cultural task indicates whither to go. The natural standards keep us on the rails.
We constantly encounter certain standards. Everyone is aware of them, and we all to some extent accept them whether we admit it or not. These universal standards apply to all people and are natural (since they are inherent in human life, not artificially imposed on it). These standards do not restrict but guide, as the tracks guide a train.
By observation and introspection we can build an understanding of these standards.
For details of these natural standards see my theory of The Actual World (Cosmology).
For the principles applying to natural standards in general see my theory of Possible Worlds (Ontology).
The sum total of natural standards is often called "the moral law", but this may suggest that it only applies to what are usually thought of as "moral" questions, when in fact it applies, through its various specific standards, to far more than that.
(Situation Ethics) "Every situation is unique, so what is right cannot be discussed in generalities."
(Ethical Subjectivism) "Every individual is unique, so nobody is in a position to judge the morality of another's actions."
(Cultural Relativism) "Every society has its own standards."
Societies apply the standards in various ways, but never deny them altogether. When did you last hear someone seriously advocating inaccuracy or treachery? However nonjudgmental we think we are, there are aspects of human conduct that cannot even be described without invoking these standards. If you doubt this, suppose that A has made a promise, and try to think of a way to tell me what happened without implying that A is bound to do something. It can't be done. If they are not bound, they did not promise. You can if you wish deny the possibility of making promises, for instance by claiming they are just predictions, but we all know the difference between a prediction and a promise; the difference is that if you promise you should fulfill.
(Interplanetary Relativism) "Your argument is based on human experience, but your conclusion requires standards that apply not only to humans but to all minds in the universe."
Human experience is all I have to go on. As mentioned before, my argument here is not conclusive, only suggestive.
(Ethical Naturalism) "There is no need to invoke standards from outside ourselves. The standards, which consist largely of fraternity, are based on our all being one family through having common ancestors. Or they are based on similarity, and if apes or computers are sufficiently similar they apply to them too."
Neither of these is sufficient to create such a standard (though similarity certainly seems to be a necessary condition). The basic problem with all such theories is that there is no reason why similarity should imply duty. We can't get an ought from an is, except in trivial cases like "it is our duty to do x". (John Searle claimed to do so in Speech Acts (1969), but he was cheating.)
(Sentimental Christianoidity) "God does not impose rules on us. Love is the only standard. No other principle is important enough to divide us."