[W]e definitely observe animals acting according to some form of natural plan.
The question then becomes: What is the nature of man? Actions that are in accordance with that nature are good, while those that go against it are evil.
I brought up the obvious problem of rape, to which he responded:
I believe that man's fundamental nature--the part of his nature that is more important than all others--is that he (or she) is capable of making complex long-range plans: We plant in the spring and harvest in the fall, or loan money at interest, or design microchips. This long-range planning is both unique to humans and our chief means of survival (as large vertebrates go, we're quite puny otherwise). Because these things constitute our nature, it is wrong to violate the essential nature of another. This would include rape, which is wrong because it violates the ability of the raped individual to act according to the plans that they have made.
I hope I've cleared things up, but please let me know if I have not. This idea is more or less a derivation of Ayn Rand, but it's easy to see in Aristotle as well--and in Henry Veatch, another great modern exponent of Aristotle.
I'm happy to go along with the Kantian idea of respecting people's rational agency. But I don't see why we should want to justify this by appealing to some notion of 'fundamental nature'.
Firstly, I just don't see what normative force this descriptive fact is supposed to have. Perhaps rationality and long-term planning are unique to humans - but so what? I don't see how it follows that we have a moral duty to cherish this particular characteristic. It doesn't strike me as obvious that it's always wrong to "violate the essential nature of another". For what if someone's "essential nature" was to hurt and exploit others: would it be wrong of us to 'violate' a parasite's parasitism? I also find implausible the implication that it's somehow wrong to go beyond the confines of our common 'nature'. So long as you don't harm anyone, why not do something new, something 'unnatural' - what's wrong with being different? Simply put: what in the world does any of this have to do with morality?
Secondly, even if we wanted to base morality on human nature, what makes 'planning' the fundamental aspect of it? This choice just seems arbitrary. Language and communication are, arguably, even more fundamental to us. We're social animals, after all. Reproduction is something else that's pretty central to many people's lives. Eating and breathing are utterly essential to all of us. Walking on two legs sets us apart from other mammals, driving in cars even more so. More abstract characteristics would include creativity, love, hate, jealousy, etc. All seem important parts of our 'nature'. Which is 'fundamental'? What does the word even mean in this context?
Will Wilkinson has an excellent post on this subject:
You and I are both part of the club of humanity because we have a shared ancestor: the first human. This, however, implies nothing about our having a metaphysically deep shared natured. Evolution works on selection over natural variation. That is, evolution works because members of a species are not homogenous. So at any time, there is simply a distribution of traits throughout a population. Maybe the distribution is a normal curve. Maybe it isn't. In any case, the distribution changes over time, and thus so do the traits of the "typical" member (if there is one). There simply is no non-contingent common core of traits that ties us together other than our shared lineage and consequent genetic similarity.
This is why I find the idea that there is a right way to live according to nature extremely dubious... We have no "deep" nature. Right now, in this neighborhood of our evolutionary history, there is a distribution of traits that one might call "typical" in a statistical sense. But this has no more deeply normative significance than would the fact that 90% of us prefer almonds over pistacchios. It makes no sense to argue that we thus ought to prefer pistacchios. People with statistically "deviant" behavioral dispositions are by definition not "normal," but their behavior is not a scintilla less "natural" than that of the normals.
As I see it, to base morality on human nature is to confuse how people are with how we ought to be (the old is-ought gap). So if you're talking about 'human nature' in a purely descriptive/biological sense, it has little to say about ethics.
But if you're talking about some special sort of extra-biological, metaphysical 'human nature', then it seems you've already smuggled in normativity. That is, one could define our 'fundamental' human nature in terms of what's morally most important about us; but it would be circular to then try to explain morality in terms of this metaphysical 'nature'.
So appealing to human nature as the foundation of morality is either fallacious or redundant. This is clearly no good either way. Have I misunderstood it?