This argument can be dismissed fairly easily. In essence it goes as follows:
- Objective morals can only come from God.
- Objective morals exist.
- Therefore God exists.
But even allowing for that excuse, there is no real excuse for Lewis simply dismissing natural explanations for morals out of hand, and there is no excuse for his leap to the conclusion that the only god on offer is his own favourite one, even if he had established his first premise.
I have blogged previously about how this mistaken model of the origin of human morality leads us to expect a pattern of human cultures which is at odds with reality in Xeno's Religious Paradox. Lewis, as a scholar in Greek, Roman and Medieval history, must have been aware of differences in morals between societies and changes in those morals over time, yet he chose to ignore that.
Although he was notoriously insular, only leaving Britain once as a WWI soldier to fight briefly in France before being repatriated wounded, he cannot have been unaware that non-Christian countries also have morals which are objectively no better or worse than those of Christian England. Unless he was assuming, as was common for the English upper classes in those days, that the English had the best morals and the cultural superiority given them specially by an English-speaking god, therefore other cultures and other moralities could be dismissed and ignored as irrelevant.
But leaving that aside, C.S.Lewis' reasoning is little more that a God of the Gaps argument - because he can't think of, or doesn't know of, an explanation for the origin and development of morals in human culture, it must have been the locally popular god.
Objective Morals Exist. The circularity in Lewis' reasoning is astoundingly obvious and breathtakingly audacious. Our morals must be objective because they come from God; they must come from God because they are objective. Come on! Really!
So, does God tell us it's wrong to hurt babies and mug old ladies because it is wrong, or is it wrong to hurt babies and mug old ladies just because God says so? If the former, from what higher authority does God derive that knowledge? If the latter, in what way is the arbitrary and capricious whim of a god objective? If morals are objective and come from a god, there must exist a standard by which this is measured and the god must be constrained by that standard. And who or what set up that standard?
In fact, in attempting to fill the gap with a god, Lewis merely moves the question back one and we are left wondering where the god gets objective morals from. An infinite regress of higher gods, perhaps? Just as the argument from first cause begs the questions, what caused the first cause? or why can the first cause argument be suspended when it suits? so Lewis' argument doesn't answer the question it purports to address.
In fact, what we have is exactly what we would expect of cultures, including moral codes and ethics, if they evolved from a common ancestor by an evolutionary process driven by local environmental conditions which included the presence in the meme pool of superstitions and notions of local or tribal gods and supernatural spirits. Like an evolving family of species, we would expect common basic features and regional differences produced by local environmental conditions, and this is precisely what we see.
None of this requires the inclusion of supernatural magic in the explanation merely to fill the gaps or, more likely in the case of C.S.Lewis' argument from morality, to arrive at the desired conclusion in order to rationalise a belief for which there was no objective evidence. And nor are we left with an infinite regress of origins and no real answer to the question.
But how well that pandered to the English desire to believe we had the best morals because we had the right god, just so long as we ignored the fact that Lewis' argument was no less valid for any god in any culture with any set of morals which they believe their god had given them and which were therefore 'objective' morals. No, it was much more satisfying to sit back in smug self-satisfaction secure in the knowledge that an Oxford don had confirmed that we were paragons of Christian virtue surrounded by countries who didn't quite measure up, and others beyond the Pale who had no idea of right and wrong and who were lucky to have us there to civilise them, as God had intended.