I'm working on Chapter 5 of Mere Christianity, and it occurred to me that Lewis is mistaken about the uniqueness of what he terms Moral Law. He notes that there is often an impulse that we ought to do good, even when we don't want to. And from that he deduces that this impulse, unlike all other impulses, must be a natural law implanted by a supernatural mind. Aside from being a humongous leap, stealthily importing a lot of assumptions not made explicit, this ignores the simple explanation of cost-benefit analysis and the difficulty of delaying gratification.
When we say, "I really ought to . . . " whether it regards going to the gym, giving to a charity, or abstaining from an extramarital affair, aren't we really saying, "I highly value an overarching or future benefit that requires this action, but there are powerful immediate benefits to not taking this action." The struggle isn't the result of a fallen nature battling with a God-given Moral Law, but simply the difficulty of turning down present goodies to obtain future ones. Remaining on the couch, keeping all your money to buy fun stuff for you, or having that smoking hot infatuation sex will all feel really good in the short term, or when we only focus on our immediate desires. But we are also aware of the wider picture, in which other (selfish!) motivations require that we do the opposite.
I just don't see why this has to rise to the level of the supernatural. Unless you are steeped in a Christian culture already. In Lewis's case, I suspect not only did he work out the "logical" steps to his conclusion AFTER coming to it, but that Christian moral teachings influenced how he looked at the issues. It might not occur to a person schooled in the Ten Commandments that adultery actually has some benefits - we all know adultery is wrong, wrong, WRONG!!!
While Lewis is really good at couching his arguments in convincing-sounding analogies, I think he fails at both perspective and imagination.