In The Abolition of Man C.S. Lewis has staked all on the assertion that what he terms the Tao (i.e. "Natural Laws" of ethics, doctrines of objective value) cannot be deduced or shown to be correct via any process of ratiocination. Lewis claims that "I am not trying to prove [natural law's] validity by the argument from common consent. Its validity cannot be deduced. For those who do not perceive its rationality, even universal consent could not prove it."
This sort of talk is absurd on its face. Either morality is teleological, or it is not. If not, an apologist such as Lewis cannot hope to have a theistic account of the Tao. If it is, we may talk about moral ends and ethical means and thereby reason out (logically and empirically) which rules are the most efficient means to those moral ends which we desire, either for their own sake or for the sake of obeisance.
Every ethical exhibit, each Earthly exemplar, one and all empirically explicable! The question one must ask is "What would happen if metaphysical materialism was really true, and the Tao naught but natural phenomenon?" Put another way, which particular moral rules would emerge organically as families, tribes and societies adopt rules motivated by their natural desires and common goals, e.g. prosperity for their kin and kith, woe unto their enemies. Lewis fails to even pose such fundamental questions because he assumes the Tao to be transcendently intelligently designed from the word go, rather than an emergent property of rational agents having increasingly complex social arrangements. Thus, Lewis begs the most interesting question which he could well have posed, that is, "What is the best explanation for the cross-cultural prevalence of certain moral principles?"
Thus, we see another great thinker stymied by making the design inference too easily and too early. In the end, Lewis has served as "only one more obscurantist," come to entreat us back to traditional morality without properly investigating either its sources or limitations.