There is no perfect mechanism for the CIA to uncover spies in its midst, or for investors to spot schemers and frauds, or for any of the rest of us to peer, clairvoyantly, inside the minds of those we do not know. What is required of us is restraint and humility. We can put up barriers on bridges to make it more difficult for that momentary impulse to become permanent. We can instruct young people that the kind of reckless drinking that takes place at a fraternity party makes the task or reading others all but impossible. There are clues to making sense of a stranger. But attending to them requires care and attention.