One of life’s great pleasures is trust: having confidence in another person to do the right thing warms the hearts of both parties. Despite the cynicism that we sometimes mistake for profundity, modern society would be impossible without a large degree of trust, including trust between people who don’t know each other. However, we all know that trust has limits. I have a legal and moral right to leave my home for a week’s holiday, jam the doors wide open, pin a sign to the gate saying “back in a...