Tripping while walking or acquiring an unjustified belief does not entail that we are in any way defective. It’s just not in human nature to walk steadily all the time—there is a tradeoff between perfect steadiness and the need to look for more distant dangers, or just the need to think about more important matters. Likewise, we sometimes need to acquire beliefs more quickly than checking the evidence carefully allows.
But a moral failure seems different. Acting immorally is always a defect. This tells us something interesting about nature: our nature has tradeoffs, but morality is never among the tradeoffs.
(A curiosity: If it is our nature never to act immorally, but it isn’t our nature never to trip, we would expect that tripping would be much more common than immoral activity. But it’s not like that.)
It is difficult, I think, to reconcile the special role that morality plays in us—the fact that moral failure is always a defect—with naturalism. On an evolutionary picture, we would expect tradeoffs to be everywhere in our nature. If we were meant by nature never to trip, we would expect to have an instinct that makes us always look down in front of us—but of course then we wouldn’t see distant danger, so instead we have a tradeoff where we scan the environment in all directions as well as looking down in front of us.
I have one worry about the above line of thought. Perhaps immoral activity is not always a defect. Maybe it is only a defect when isn’t innocently ignorant. Think of the extremely difficult cases that come up in medical or military ethics where one needs to act very quickly. In those cases, there just isn’t enough time to always figure out what the right solution is, and it does not seem to be necessarily a defect when the conscientious doctor or officer acts immorally, if she has given the matter as much thought and are as there was time for and innocently done what appeared right.
Maybe that’s right. It’s still surprising on a naturalistic picture of our nature and origins that it is always a defect to act knowingly immorally. We would expect our nature to exhibit tradeoffs even there.
And maybe the innocent ignorance cases aren’t a problem. Maybe in such cases, the action is to be described in terms that makes it right, like: “Performing an operation that after the due amount of investigation appeared most in keeping with the salient goods.”
Of course, some naturalists simply deny that there is any coherent concept of “defect” to be applied to us. The above line of thought may be grist for their mill. Another view might be to bite the bullet and say that we are riddled with tradeoffs through and through, and it is no defect when we occasionally act immorally. On the contrary, sometimes (say, when it causes great harm to one’s genes getting passed on) acting morally is a defect—but we should be guided by what is right or wrong, not by what is or isn’t a defect in our nature. This latter view is not, I think, very popular, but one finds it in Andrea Dworkin’s idea that “the God who doesn’t exist” (nature? evolution?) has designed us so that until recent times we could only reproduce by means that, according to Dworkin, are innately oppressive to women.