I first learned to think and do philosophy as part of a tradition, a fairly conservative Catholic tradition that took both Aristotle and Aquinas seriously. My graduate training was very different, but the effects of that initial training are still in me. I might be a very bad and marginal member of that tradition, but I am still in it in some ways. One way that I remain is in a general respect for virtue theory and—oddly enough for an avowed homosexual—natural law approaches to ethics. I still take Aristotle and Aquinas seriously and I think their approaches to the good life, to flourishing, to what is good for us still speak to us.
If there is something odd about this it is because natural law moralists, in particular, have been at the forefront of objections to the decriminalization of homosexuality and to recognition of same-sex marriages. I'm not interested in debating whether modern advocates of natural law are in the right here. But I do want to note one important thing. Both virtue theory and natural law theory are meant to be empirically grounded theories. They make pronouncements about what is good for beings like us and what would amount to a good life for beings like us based on facts about our biology and psychology.
It is a basic assumption of both sorts of accounts that humans have some immutable nature. I think this is probably right, at least in the medium term; what might happen to that nature over evolutionary time is a different issue. But, many modern proponents of each of these theories seem to assume that our knowledge of this nature is also immutable. What I mean is this: Contemporary natural law theorists operate under the assumption that Aristotle and Aquinas had a complete and completely correct account of human nature, in its biological and psychological aspects. Thus, they believe not only that human nature is immutable, but that we have known all there is to know about it for at least almost a millennium.
What we have learned about human biology and human psychology and the nature of human interactions since the middle ages is or seems to be of almost no interest to many practitioners of both virtue theory and natural law ethics. You see them quoting Aquinas as authoritative on all such matters.
Now, I think that Aquinas understood quite a bit about human psychology, but I don't think he got it all. And, his biology was pretty bad. Similarly, I think Aristotle understood human motivation and psychological development and society pretty well, but he was missing out on some pretty important pieces, pieces which have partially been supplied by further exploration in the ensuing years.
Mostly, I think that Aquinas and Aristotle and others in this tradition were right to base an ethic on what we are like and what will lead to happy and fulfilling and flourishing lives for creatures like us. They were also right to think that is largely an empirical question. But this empirical question is an empirical one, not an a priori one or one that was settled in the high middle ages. If we discover new things about ourselves—say about sexuality or human interaction or the family—then our theory has to respond to that.
If virtue theory or natural law is just a constant rehashing of what people thought 900 or 2300 years ago, it isn't philosophy, it isn't even virtue theory or natural law, it's just a dead orthodoxy. And, that's exactly how it should be treated: as dead and irrelevant.
(I should mention one exception to the immutability of the theory here: Almost all such theorists have discovered that lending at interest is morally acceptable; that we learned something about economics that Aquinas wouldn't have known, since he roundly condemned this practice as usury and a violation of the natural law. This exception may be self-serving or might be a realization that the theory needs to evolve.)