If the Moral Law was one of our instincts, we ought to be able to point to some one impulse inside us which was always what we call 'good,' always in agreement with the rule of right behaviour. But you cannot. There is none of our impulses which the Moral Law may not sometimes tell us to suppress, and none 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. But there are situations in which it is the duty of a married man to encourage his sexual impulse and of a soldier to encourage the fighting instinct. There are also occasions on which a mother's love for her own children or a man's love for his own country have to be suppressed or they will lead to unfairness towards other people's children or countries.
Strictly speaking, there are no such things as good and bad impulses. Think once again of a piano. It has not got two kinds of notes on it, the 'right' notes and the 'wrong' ones. Every single note is right at one time and wrong at another. The Moral Law is not any one instinct or set of instincts: it is something which makes a kind of tune (the tune we call goodness or right conduct) by directing the instincts.
C.S. Lewis, Mere Christianity, Chapter 2 (1952)