Now how should we decide between these ethical views? To my mind, the fact which is of primary importance is the one we started with, that we all have a deep-seated primary belief in the reality of Ethics. Even people who say there's no such thing as ethical truth suddenly sound quite different when somebody treats them unfairly. And cultural relativists look down on their ancestors who persecuted other cultures, and comment on how much moral progress there's been since then, showing that they actually believe in moral relativism for moral reasons. Some ethical framework seems to be embedded as axioms in the human mind.
It's no good to argue that ethics must be subjective because different cultures disagree about it. People disagree about all sorts of things, many of which are quite real. And there are various ways people can be rationally persuaded to change their ethical views; that's how moral progress happens.
Nor is Darwinian Evolution fatal to the idea that we know ethical truths. No sensible Darwinian says that our knowledge of e.g. Mathematics or Biology is necessarily unreliable just because our capacities were developed through Natural Selection, since that would refute the Theory of Evolution too! We are not here concerned with the origin of our moral ideas, but with their truth. The origin of human ethics (which is lost in the mists of prehistoric time) would be relevant only if it implied that the ideas are invalid. But this would not follow, simply from the fact that our ethical views have an origin. In general, Darwinian evolution gives us true beliefs, not false ones, since for the most part the ability to acquire knowledge about the world is adaptive. In order to prove that our moral beliefs are unreliable, we would have to show that they originated in a way which was completely disconnected from their truth. Any such argument would involve a whole raft of controversial philosophical assumptions, not to mention the speculation common to all Evolutionary Psychology arguments. Morality leaves no fossil record. Although it is certain that our ethical capacities have some historical origin, we are in a far better position to assess what it means to be a human being today, then to speculate about these origins.
But it may be felt that Ethical Nihilism follows automatically, from the fact that right and wrong are not mentioned anywhere in the Laws of Physics. Naturalism, you see, is the attempt to reduce all realities down to those described by the Natural Sciences. Anything which doesn't fit gets cut out or else stretched to fit, as on the bed of Procrustes. In my view, this is not a benign use of Occam's razor. Instead it is a zealous oversimplification which throws out nearly all the realities of experience, in order to save a theory that won't cover them.
We have already seen how very similar reductionistic arguments would rule out Consciousness, but in that case we know the conclusion is false. If this type of reductionistic argument fails so spectacularly in the one case where we can really check it, why should we give it any credence when it is deployed as an argument against morality? (Or the existence of aesthetics, free will, personal identity, or whatever is supposed to be eliminated next.) You could even say that, since I believe in the existence of good and bad because they flavor my experiences, the mystery of Consciousness and the mystery of Ethics are intimately connected to each other. Both are features of reality which I could never have derived from a purely literal intepretation of the physical facts.
Some Naturalists believe it is possible to derive ethical laws from the physical sciences, but this is a rather tall order. It runs into the famous Is-Ought problem, articulated by David Hume, who highlighted the logical difficulty in deriving an ought statement from any number of purely factual, nonmoral statements. (Hume himself believed that morality was just a fact about human sentiments towards certain actions, an example of a subjective view.) Some rather problematic attempts to construct a purely Natural system of Ethics are reviewed here:
Of course, part of the problem is that the perfect division between is and ought is an artificial distinction in the first place. In our actual experiences, the two are nearly always joined together. We do not experience the world dispassionately. The vision of a world of pure facts is obtained by abstraction. It is obtained, not so much by eliminating half of our experiences, but eliminating half of each experience, the part of ourselves which cares about what we are seeing. This abstract representation of reality may be very useful for certain scientific purposes, but the map is not the territory. If we are unable to recover certain aspects of our experience from the map, it means that the map is incomplete, not that those experiences are invalid.
The brain is a very complicated organ which tells us a great many things about the world. Some parts of it allow us to deduce scientific facts, while others deliver to us ethical truths. To my mind, it is irrational and capricious to reject all those aspects of our thinking except that very limited set which we use when formulating physical laws (and even there, our sense of beauty plays a role). Rather, the fundamental deliverances of our brain ought to be accepted by default unless we have good reason to reject them. That is undivided looking: thinking with our whole mind.