In 1947 a federal appeals court decided a case about a barge, the Anna C., that broke loose in New York harbor while its bargee was ashore. Judge Learned Hand wrote that the owner’s duty to guard against such accidents “is a function of three variables”: the probability that the barge will break away, the gravity of the injury if it does, and the burden of adequate precautions.
He then wrote it as algebra. “If the probability be called P; the injury, L; and the burden, B; liability depends upon whether B is less than L multiplied by P.” It became known as the Hand formula.