If our moral intuitions are survival instincts resulting from evolutionary processes, why do we feel obligated to do things that we don’t have any desire or impulse to do? Or why do we feel obligated to side with our weaker impulses? As C.S. Lewis observes, our impulses are something quite different from the standard by which we measure them.
That standard that we sense is moral law. It often obligates us to act in ways that have nothing to do with our own interests or survival. Animals make decisions and act according to their survival instincts. But we’re different—we often act according to moral obligation. Perhaps instincts can be explained by evolutionary processes. Moral obligation cannot.