St John Paul II singles out St Paul VI’s teachings on the intrinsic evil of contraceptive practices: “‘Though it is true that sometimes it is lawful to tolerate a lesser moral evil in order to avoid a greater evil or in order to promote a greater good, it is never lawful, even for the gravest reasons, to do evil that good may come of it…”
The conscience is like the captain of a ship who needs maps and reference points to navigate safely. The conscience cannot invent its own rules of morality. It needs a stable reference point independent of any individual’s whim or caprice. It needs to know what is truly good and what is truly evil so that it can judge correctly.
Another reason why it is called “Natural Law” is that it can be known by the natural light of reason (without the aid of faith). St Paul says that even the Gentiles have this law “written on their hearts” (Romans 2:15).