The second chapters deals with the foundation which Lewis is trying to build (the Law of Nature, Right and Wrong, Morality), and some objections to these claims.
The Moral Law tells us the tune we have to play: our instincts are merely the keys.
There is none of our impulses which the Moral Law may not sometimes tell us to suppress, and none of which it may not sometimes tell us to encourage. It is a mistake to think that some of our impulses - say mother love or patriotism - are good, and others, like sex or the fighting instinct, are bad. All we mean is that the occasions on which the fighting instinct or the sexual desire need to be restrained are rather more frequent than those for restraining mother love or patriotism.
If no set of moral ideas were truer or better than any other, there would be no sense in preferring civilised morality to savage morality, or Christian morality to Nazi morality. In fact, of course, we all do believe that some of the people who tried to change the moral ideas of their own age were what we would call Reformers or Pioneers - people who understood their morality better than their neighbours did.... The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other.
I conclude then, that though the difference between people's ideas of Decent Behaviour often make you suspect that there is no real natural Law of Behaviour at all, yet the things we are bound to think about these differences really prove just the opposite.
Source: Mere Christianity c. 1952 C.S. Lewis