"The preservation of society, and of the species itself, are ends that do not hang on the precarious thread of Reason: they are given by Instinct. That is why there is no need to argue against the man who does not acknowledge them. We have an instinctive urge to preserve our own species. That is why men ought to work for posterity. We have no instinctive urge to keep promises or to respect individual life: that is why scruples of justice and humanity—in fact the Tao—can be properly swept away when they conflict with our real end, the preservation of the species. That, again, is why the modern situation permits and demands a new sexual morality: the old taboos served some real purpose in helping to preserve the species, but contraceptives have modified this and we can now abandon many of the taboos. For of course sexual desire, being instinctive, is to be gratified whenever it does not conflict with the preservation of the species. It looks, in fact, as if an ethics based on instinct will give the Innovator all he wants and nothing that he does not want."
C.S. Lewis, The Abolition of Man