a serious ethical theory must pass three tests:Lord Keynes has a good starting point, but he* omits some important considerations.
(1) it must not commit the “appeal to nature” fallacy;
(2) it must explain how it overcomes or is consistent with G. E. Moore’s “naturalistic fallacy,” and
(3) it must explain how it overcomes or is consistent with Hume’s “is–ought” problem (sometimes called Hume’s Law and Hume's Guillotine). [Links original]
*I'm presuming that "Lord" is meant in a gendered sense.
First of all, a naturalist such as myself must first ask: what facts does the theory try to explain? This question lies, I think, the core of the opposition to G.E. Moore's non-naturalism. Although I haven't made a detailed study of Moore's ethics, he(and ethical non-naturalism in general) seems to assert that ethics by definition cannot simply be an explanation of some facts. Instead, ethics is the study not of how the objective* world actually is; ethics is the study of how the world ought to be and therefore is actually not. By definition, we cannot differentiate naturally between all the different ways the world is not. There is no natural, scientific truth about how the world objectively ought to be, i.e. one preferable way (or a definable subset of ways) the objective world is not, because there are no observable facts about different ways the world is not that a scientific theory could explain.
*I mean "objective" here in the sense of including only the world outside our minds.
Although there's no metaphysical basis for believing that natural science has a monopoly on truth or knowledge, we should not blithely assume that even if we eliminate science a priori that there is a non-scientific truth to be found. Assuming arguendo that science does give us truth*, my first response to someone placing a domain of discourse outside the bounds of scientific truth is to then ask why we should conclude there is any truth at all in that domain. If science is ruled out, why should we then believe there is any such thing as non-scientific ethical truth? Even if we have some intuition that there really are ethical truths, we already know our intuition is a fallible guide; we need a more rigorous demonstration that there really is something out there to be found in some nonscientific, non-naturalistic way.
*Denying this assumption does not immediately help the case for alternative kinds of truth or knowledge.
The case for non-naturalistic ethics seems similar to a particular apologetic strategy of placing God outside the bounds of natural knowledge, and then assuming the existence of a God. The response is similar: to place God, or anything else, outside the bounds of scientific knowledge makes the assertion of its existence, and even meaning, suspect.
If you were paying attention, you saw me sneak in a subtle bias above, when I talked about objective reality, i.e. the world outside our minds. But, of course, the subjective is also real, and we have ample natural, scientific, observational evidence to conclude that minds really do exist, and enough evidence to conclude that our minds are an emergent physical property of physical brains*. And, it seems, there are some interesting facts to explain about minds, facts that seem to bear a striking resemblance to the sorts of things philosophers label as "ethics."
*Like all scientific theories, this conclusion is provisional, and does not exclude the possibility that we might have evidence in the future to radically revise our understanding of minds.
There are some observable facts about our minds: human beings have preferences, we live socially, and our societies are the result of dialectical processes, the most notable of which are the dialectics between selfishness and altruism (both terms broadly construed) and between social norms and individual desires. And indeed, ethical philosophy concerns itself with these dialectics. Some ethical philosophies take a more reductive view and simply declare either selfishness or altruism, or either social norms or individual desires, irrelevant, impossible, or evil; most nuanced philosophies acknowledge these dialectics at least as tensions, and usually try to resolve the tensions analytically.
Ethics takes its normative character from the fact that human beings are not merely passive, sphexish objects moved around by social reality, but rather actors, beings that act on both the physical and social world to satisfy their preferences and achieve their goals. Furthermore, society presents tensions and dialectics: we cannot satisfy many of our preferences without a society, but society actively frustrates our preferences (see e.g. Freud's The Future of an Illusion and Civilization and Its Discontents.) Fundamentally, then, the study of ethics in the purely descriptive sense consists of psychology, sociology, anthropology, and, to some extent, political science and economics. How have people historically worked out the dialectic between selfishness and altruism, and between social norms and individual desires? But as noted before, people are not just passive objects, they are actors, operating in the present. So we can talk — still scientifically, still naturally, because people and their minds actually exist — about all the different things people want now, and how we can negotiate to decide some reasonably coherent set of goals for the future, and set about achieving them.
Looked at in this way, most of the confusion about ethics seems to disappear. The relevant facts are precisely those subjective facts about how human beings want to change both the physical and social world. The constraints are first how the physical world actually is, and how our actions effect it, and second, how our physical and cultural evolutionary history interacts with our preferences.
I tend to call this view pragmatism: the nontrivial study of how human beings can most effectively change the physical and social world so as to satisfy our preferences and goals. Pragmatism is a fundamentally consequentialist, i.e. concerned primarily with outcomes. Pragmatism is also, in a loose* sense utilitarian, concerned with the promotion of human "happiness" and the reduction of "suffering." (Note that pragmatism should not be confused with expediency or populism, both of which select out some features of normative standards, either immediate effect or majority opinion, as definitive and exclusively authoritative. Pragmatism holds that all the consequence, for all people, are relevant, even though sometimes the immediately useful or the majority opinion might be the pragmatically best option.)
*i.e. without the Benthamite commitment to a strictly linear notion of suffering/happiness and a strictly additive notion of aggregate happiness, which seems like a technical flaw in Benthamite utilitarianism, not a fundamental flaw.
There are, of course, objections to pragmatism. First pragmatism is well, not something other than pragmatism. If one is committed a priori to non-naturalism, then pragmatism is, by virtue of its naturalism, unacceptable. If one is committed to deonticism, pragmatism is, by virtue of its consequentialism, unacceptable. If one is committed to objectivism, then pragmatism is, by virtue of its inherent subjectivism, unacceptable. Pragmatism is what it is, however, and to simply say that it's wrong because it's not an alternative theory is to beg the question at some level or another.
Sometimes these objections are subtly disguised. For example, one might argue, for example, that if the vast majority of people wanted slavery, and very few people objected to it, then pragmatism would condone slavery. The pragmatic response is that the vast majority of people don't want slavery and most do object to it. The opponent believes here that slavery is objectively wrong. But what is the basis for this conclusion? If it is the present unpopularity of slavery, then the evidentiary basis for both our theories would be contradicted by nature. If the opponent believes slavery is objectively wrong for some other reasons than present unpopularity, then she has the burden of explicating those reasons in a non-question-begging way.
Another objective to pragmatism is that it does not provide any guide as to what we should prefer; pragmatism takes our existing preferences, as well as the mutability of those preferences, as facts about the world. But this objection again begs the question: why should an ethical theory tell us what we should prefer, instead of giving us guidance on how we can negotiate and achieve our preferences in a social context? Simply that one prefers than an ethical theory guide our preferences is committing the same "sin" of taking his preferences for granted that he ascribes to pragmatism.
Fundamentally, pragmatism at least satisfies Lord Keynes requirements. It does not commit the "appeal to nature" fallacy, nor does it commit the "is-ought" fallacy, because it does not take the natural world, the world as it is, as the ethical ideal: it recognizes that people want to change the world. It makes an end-run around Moore's naturalistic fallacy, by simply declaring Moore's meta-ethics as incoherent; pragmatism is naturalistic, but takes as its primary domain the subjective, not the objective world.
Thus, I am a pragmatist.