While Lewis is making a very a good point about our relative/analogical knowledge of good and evil, he happens to do so by trading on a widely-popular caricature of the doctrine of total depravity.
Most people are introduced to the teaching by hearing something along the lines of, “That John Calvin, he was such a pessimist. Did you know that he taught that we were totally depraved? That all of us are about as awful as it gets, none of us knows right from wrong, and we’re born simply and utterly wicked? No wonder he was a jerk.” Or something like that.
The problem is, that is neither what total depravity, properly understood, nor John Calvin teach with respect to human nature.