CS Lewis on the idea that morality is nothing but instinct, taken from Book 1, chapter two, of Mere Christianity: “Some Objections

For example, some people wrote to me saying, "Isn't what you call the Moral Law simply our herd instinct and hasn't it been developed just like all our other instincts?" Now I do not deny that we may have a herd instinct: but that is not what I mean by the Moral Law. We all know what it feels like to be prompted by instinct-by mother love, or sexual instinct, or the instinct for food. It means that you feel a strong want or desire to act in a certain way. And, of course, we sometimes do feel just that sort of desire to help another person: and no doubt that desire is due to the herd instinct. But feeling a desire to help is quite different from feeling that you ought to help whether you want to or not. Supposing you hear a cry for help from a man in danger. You will probably feel two desires-one a desire to give help (due to your herd instinct), the other a desire to keep out of danger (due to the instinct for self-preservation). But you will find inside you, in addition to these two impulses, a third thing which tells you that you ought to follow the impulse to help, and suppress the impulse to run away. Now this thing that judges between two instincts, that decides which should be encouraged, cannot itself be either of them. You might as well say that the sheet of music which tells you, at a given moment, to play one note on the piano and not another, is itself one of the notes on the keyboard. The Moral Law tells us the tune we have to play: our instincts are merely the keys.

Another way of seeing that the Moral Law is not simply one of our instincts is this. If two instincts are in conflict, and there is nothing in a creature's mind except those two instincts, obviously the stronger of the two must win. But at those moments when we are most conscious of the Moral Law, it usually seems to be telling us to side with the weaker of the two impulses. You probably want to be safe much more than you want to help the man who is drowning: but the Moral Law tells you to help him all the same. And surely it often tells us to try to make the right impulse stronger than it naturally is? I mean, we often feel it our duty to stimulate the herd instinct, by waking up our imaginations and arousing our pity and so on, so as to get up enough steam for doing the right thing. But clearly we are not acting from instinct when we set about making an instinct stronger than it is. The thing that says to you, "Your herd instinct is asleep. Wake it up," cannot itself be the herd instinct. The thing that tells you which note on the piano needs to be played louder cannot itself be that note.

Here is a third way of seeing it 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 any set of instincts: it is something which makes a kind of tune (the tune we call goodness or right conduct) by directing the instincts.

By the way, this point is of great practical consequence. The most dangerous thing you can do is to take any one impulse of your own nature and set it up as the thing you ought to follow at all costs. There is not one of them which will not make us into devils if we set it up as an absolute guide. You might think love of humanity in general was safe, but it is not. If you leave out justice you will find yourself breaking agreements and faking evidence in trials "for the sake of humanity," and become in the end a cruel and treacherous man.