With respect to Lewis’s moral argument, poses some questions about how Lewis structured the moral argument, but I think that we could consider his argument by using the structure provided by C. Stephen Evans:
1) Probably, unless there is a God, there cannot be objective moral obligations.
2) There are objectively binding moral obligations.
3) Therefore, probably, there is a God.
I realize that some of Lewis’s rhetoric suggests something stronger than the probabilistic argument I present here, but part of the responsibility of a philosopher succeeding Lewis is to develop and strengthen his arguments and to make reasonable conclusions as to what the arguments really do show.
An important part of Lewis’s defense of the moral argument is his argument for objective moral values. However, it is important to point out that Lewis had a considerable interest in defending moral objectivity per se. Hence, his book The Abolition of Man was a critique of subjectivist philosophy, particularly as it manifested itself in the underlying presuppositions of English textbooks. He criticized Lewis in the previous edition for comparing moral values, for the subjectivist, to a mere, private taste of his own, such as a liking for pancakes or a dislike for spam. In this edition he does admit that Bertrand Russell compared moral values to a taste for oysters, and maintained that there could be no debate concerning moral values. This we might call simple subjectivism. However there can be a more sophisticated type of subjectivism, and here he turns primarily to the philosophy of Hume to provide a foundation for the kind of subjectivist view that he finds unrefuted by Lewis’s arguments.
Indeed, if someone were to try to generate a moral system without appealing to anything which might embarrass a modern naturalist ontologically, such as Kantian synthetic a priori knowledge, Platonic forms, Aristotelian entelechies, or the Christian God, it’s hard to improve on Hume, at least as a place to start. The moral life, for Hume, rests on two pillars: social utility and sympathy. First, human beings have an interest in getting along peaceably with their fellows. Second, we all have sympathetic feelings toward others. Some feelings are private feelings of our own, such as when we call a certain person “my enemy.” If I have personal feelings of enmity toward someone, it may be simply that he and I are rivals for something that we cannot both have but which I desperately want. But if I call him vicious, Hume says that I am expecting other people to concur. So if I were to call, say, a serial killer vicious, I expect the sympathy of other persons toward his victims and their families to bring them to the same conclusion that I have drawn.
But I think there are some problems which attend Humean subjectivism which also attend other forms of subjectivism that say “Yes, moral judgments are subjective, but, they have characteristic X that puts them a cut above other subjective feelings. First, there is a social utility to much behavior that we call moral, but the social benefits of the moral life are rather contingent. If I have long ago committed a murder, and am living peacefully in another state, I can effectively ruin my social life by confessing the crime, and yet that is precisely what I ought to do. Slavery was pretty socially useful in undergirding virtually every society from the Egypt of the Pyramids to the antebellum South, but it is nevertheless a morally unacceptable practice.
At the same time, Humean sympathy seems to be one more emotion amongst many. Other people may find it agreeable to deal with people who have lots of sympathy in their emotional makeup, as opposed to, say, sadism, by why are these feelings thought to be superior to others. The following quote from Lewis on Instinct could also be used as a response to Hume’s attempt to ground morality on sympathy:
But why ought we to obey Instinct? Is there another instinct of a higher order directing us to do so, and a third of a still higher order directing us to obey it?—an infinite regress of instincts? This is presumably impossible, but nothing else will serve. From the statement about psychological fact 'I have an impulse to do so and so' we cannot by any ingenuity derive the practical principle 'I ought to obey this impulse'. Even if it were true that men had a spontaneous, unreflective impulse to sacrifice their own lives for the preservation of their fellows, it remains a quite separate question whether this is an impulse they should control or one they should indulge. For even the Innovator admits that many impulses (those which conflict with the preservation of the species) have to be controlled. And this admission surely introduces us to a yet more fundamental difficulty.
Hume appeals to the universality of sympathy as a basis for the moral life, but does sympathy alone possess this universality? Competitiveness, the desire to get ahead of the other guy, is, so far as I have been able to tell, pretty universal in the human frame as well. (Drive a few miles in rush hour traffic and ask yourself whether sympathy or competitiveness has a greater purchase on human nature). Our morality is supposed to step in and act as a referee when we are pulled by two opposing impulses. So I am driving down the road on the way to an important meeting, and I see an injured person lying in the road. I want to get to the meeting, because a promotion hangs on my being there, and I want to climb the ladder at my job. But the person in the road needs my help. Yes, my sympathy is on the side of stopping by the side of the road, but I also want that promotion. Why should I obey my sympathy as opposed to my desire to get ahead? I don’t think Hume’s theory, or any other subjectivist theory, provides a good answer.