While Lewis is the most elegant on this issue, William Lane Craig provided perhaps the cleanest logical defense of the idea by use of a simple Modus Tollens syllogism:
Premise 1 - If God does not exist, then objective morality does not exist.
Premise 2 - But objective morality does exist.
Conclusion - Therefore, God exists.
It surely is curious that the average skeptic relies so heavily upon the "problem of evil" to assail the Christian argument when evil as a concept only has meaning if there exists some objective basis for determining what is evil and what is good. In a purely materialistic universe, what could it mean to complain about "evil?" Evil in such a world would be nothing more than our cultural labeling of certain actions we simply do not prefer. In other words, we could never call one animal group (say, Al Queda) any more or less moral than any other. They think we are "evil" and we think they are "evil." But there is nothing outside these two cultures that might provide some meaningful arbitration between the two opinions.
Thus, the syllogism is enriched by Dr. Craig as follows:
Premise 1 - If God does not exist, then objective morality does not exist.
Premise 2 - Some things are really evil.
Premise 3 - But that entails that objective morality exists (in order to determine what "evil" and "good" are).
Conclusion - Therefore God exists.
So, in the end, the existence of evil in the world is another proof of God's existence, for what can evil mean without God's existence.
And now a summation of Lewis' argument:
Lewis' basic argument is that we carry with us a shared internal law of right conduct. We feel this "law of human nature" constantly pressing on us, and it seems clear that none of us made it. It is something that is a feature of the world, or the natural structure of the world, that we merely discover and do not invent. Lewis feels that the best accounting for this is a supreme mind behind the universe who made us this way, to reflect His/Its own moral character. But none of us keep this moral law, and so we put ourselves at odds with the power behind the universe.
There are, of course, various challenges to this position, and Lewis answers each in turn.
Objection 1 - There are so many different moral systems, so there must not be an objective foundation to the moral law.
Lewis' response here is sufficient. He maintains that perhaps the idea that there is no objective morality does not follow from a mere observation of differences in moral systems. Perhaps the differences are minor in nature, at least in comparison to the striking similarities. Lewis asks us to consider the ancient moral systems of the Babylonians, Greeks, Persians and the like, and in really looking into them one is struck by how alike they are to each other rather than how dissimilar they are.
In essence, Lewis contends that all moral systems have crucial similarities. For example, one is hard pressed to think of a culture in which men are praised for running way in battle. Selfishness is never admired either. It is as if Lewis is suggesting that all cultures have the same moral infrastructure (foundation and pillars) and then they build around these structures to their own liking. For example, all cultures would claim that fidelity in marriage is a good thing, but some cultures allow for 1 wife, some 4 wives and others even more. Simply treating a woman however you want to treat her is not acceptable in any culture, etc.
An additional point can be raised to defend the biblical view here. Christianity acknowledges the reality of moral erosion over time. It begins with individuals ignoring conscience on certain matters. A classic example in the high school context would be cheating. There are many students who have decided to ignore their consciences on this question. If one does this long enough, conscience will become a blunted instrument. There will be an internal loss of moral sensitivity to cheating. It is also unlikely that such a person will surround himself with people who find cheating morally reprehensible and face their criticism. Instead, he will surround himself with other cheaters. If he does this, he will have lost any internal sensitivity to moral law on the question of cheating, but he will also have lost any external points of reference to the moral law, because he is now isolated from those people who might remind him of his moral responsibilities. I frankly think this process is a sufficient explanation for the divisions of culture and subculture in our world. Look at the gay subculture and you will see this same process. Look at "pot-heads" in high school and again you will see this process of ignoring conscience and then isolation with others who have done the same thing on the particular moral issue in question.
Objection 2 - Moral Law is merely put into us by parents and teachers.
Two responses here are sufficient. Lewis points out that it is silly to suggest that just because something is taught to us, it must be a human invention. We are taught math and logic this way, but no thinking human being would suggest that the core principles of these subjects were made up by men, as if they could be anything we want them to be .
But perhaps the most important response is that this idea is philosophically inane. It amounts to believing that there can be no moral progress and no moral exemplars. It forces one to accept the conclusion that what we believe about right conduct is a simple feature of our cultural environment. If what we believe to be moral comes directly from our culture, then if we were raised in Nazi Germany, we would default to the view that it is moral to hate Jews, etc. The death knell for this objection is that relativism in morality results in nothing more than the rule of the stronger. One loses any objective basis to complain about radical Islam, the Nazi's or any other hated moral position. The most one can ever say is that "we don't like it," which of course will be irrelevant if one lacks sufficient firepower to do something about it.
Objection 3 - Moral law is nothing more than a feature of our evolutionary development, an instinct making "herd survival" possible.
We hear a lot on this in our day, and the reason we do is that the secularists among us are trying desperately to find some objective basis for morality without God. Of course, nature is the only candidate, and so morality becomes some hidden feature of our genetics. Bats develop sonar and we have this super-instinct we call "morality."
A lot can be said in response to this. For his part, Lewis offers only one rather elegant argument. Essentially, he says that if in fact morality was an instinct, we would find that one of our instincts was always aligned with the moral law, but that is not what we experience. Instead, we experience the odd phenomenon of recognizing that sometimes our instincts are to be encouraged and sometimes suppressed. For example, our instinct for sex is sometimes to be encouraged and sometimes suppressed, depending on time and context.
If we saw a person drowning, we would face conflicting instincts; one for self-preservation and the other for "herd survival." What are we to do? Lewis thinks it too simplistic to suggest that we do what our instincts tell us to do because we are facing conflict between instincts. There is a third voice that comes in and tells us, in this case, to encourage the weaker of the two instincts and save the man. But of course the thing commanding instincts cannot be an instinct.
Secondly, we must ask why it is obvious that a "super-instinct" is sourced in genetics? It is clear that genetic information codes for the production of proteins, but is it clear that it codes for the production of beliefs? And even if it were clear, we would still encounter the problem generally faced by geneticists--namely, the source of genetic information in the first place. The only naturalistic solution is the chemicals themselves (amino acids and the like). It is maintained that they had to randomly arrange themselves into spectacularly encyclopedic configurations on their own. In short, chance is again trotted out as the explanation for all genetic information. But if that is the case, then the genetically coded beliefs we call "morals" are also artifacts of chance. What of course makes this whole idea credulous is the nature of the information systems we are talking about. Both genetics and morality are information rich systems that govern biological and social life. This information appears to be logically anterior (foundational) to biological and social systems working. For the Christian, information indicates intelligence, plain and simple. The nature of these systems requires a God who could compose nature with these information systems embedded.
Thirdly, and related to this second point, we must ask why it is imperative to obey an "instinct" that is the product of a random material process alone. Fine, my moral instinct is telling me to save some kids life, but why should I care to obey it? It is not as if it is "best" for my nature. How can I know what is best for my nature if my nature is produced by a mindless, purposeless process? We all know that the state can make as many laws as it wants to, but if it cannot enforce them, they are largely meaningless. In the same way, I can call any one of my instincts a law created by evolution, but evolution cannot enforce it, so there is no reason for me to listen to my instincts. Perhaps I choose to kill a man because he listens to bad rap music, in defiance of my "moral instinct." Even if I am caught by the authorities and sentenced to die, I can still maintain that it is my right as an evolved being to deny any of the supposed instincts to which I am allegedly bound, and to do so in experimental boldness.
Fourthly, and as a continuation of the previous point, perhaps in asserting my freedom to live counter to my moral inclinations, I am actually more evolved than someone else. Someone may claim that I have social responsibilities as an evolved animal, but why? If I choose to deny social responsibilities, perhaps that will be the key to the expression of my utmost individuality. Perhaps what is needed for total self actualization is the denial of others. Perhaps the sociopath is the most evolved human animal. Why should we conclude otherwise? If the atheist is going to condemn the sociopath, it seems to me he will have to do so on evolutionary grounds. He is going to have to say that "most" humans have this sense, just as most cheetahs have 4 legs for optimal speed, and so the "moral sense" is the "will of evolution." But evolution is arbitrary, causing unpredictable reconfigurations of genetics and various changes in the population. What if we came across a cheetah that couldn't run very well, but was clever enough to kill the other cheetahs while they slept? We might be tempted to say that the clever cheetah is broken because it wants to kill other cheetahs, but are we going too fast (pardon the cheetah pun)? If evolution really is a random process, then who are we to judge what it should produce? It seems to be rationally plausible that the pure isolationist sociopath could be the next step in evolution. And even if he isn't, the only way to condemn him, provided naturalism is true, is not to say that he is wrong, but that he is "unacceptable to the pack."
One final peculiarity about moral law: Lewis points out that it is strange that other laws of nature consist in simply observing how nature regularly behaves. But moral law is different. It is equally objective, but tells us not observational data about how things are, but rather provides ongoing critical interpretation of how things ought to be. This is Lewis' way of addressing what ethicists refer to as the "is/ought" dilemma. Morality is not merely experimental, discovered and evolved over the long course of human history. It is rather something by which we evaluate all of human history, and it is only a moral law that is not produced by random changes over time that can give us some meaningful reason to believe that what we have now in moral systems is better than the systems of our distant ancestors. Indeed, often we look at the ancients and note that they had things right in various ways from which we have departed. But note well that this thing that is used to evaluate historical evolution or devolution is not itself the product of evolution.
In short, the atheistic system can give us observation of moral systems--the "is" of ethical language. What atheism cannot give us is any meaningful foundation for "ought" language. It reduces to something like, "person A in circumstance A1 killed person B." But then the atheist goes on to note that, "person C in circumstance A1 did not kill person B." We are tempted to evaluate the circumstance and conclude that one was right and the other wrong, but how can atheism provide any such conclusion?
