It does not follow that because the moral instinct does not always win, the moral impulse is not instinctive. It could be that different instincts are stronger in different situations. You would have to demonstrate that this is never the case.
Lewis does not explain how all instincts would be right if morals were instinctive. He merely declares it.
I’m worried that because you were only able to quote the argument back at me without explaining it, you haven’t really understood it. That’s my gripe about pasting things back and forth.
His moral argument depends on declaring a lot of things without explanation. Because it’s an inherently terrible argument. I would venture to guess that the audience for Mere Christianity would have taken all his premises without questioning them at all.
perhaps some more explanation by lewis is warranted:
"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."
-CS Lewis, Mere Christianity
Lewis was a former atheist, turned Christian-
"CS Lewis's credentials: Lewis began his academic career as an undergraduate student at Oxford University, where he won a triple first, the highest honours in three areas of study. He was then elected a Fellow of Magdalen College, Oxford, where he worked for nearly thirty years, from 1925 to 1954. In 1954, he was awarded the newly founded chair of Medieval and Renaissance Literature at Cambridge University, and was elected a fellow of Magdalene College." - Wikipedia