Paul Samuelson was once challenged by the mathematician Stanislaw Ulam to name a single proposition in all social science that was both true and nontrivial. Samuelson thought of a good answer: the principle of comparative advantage.
He died today, age 94. As Ricardo Caballero puts it, “We will miss him and his unmatched wit very much, but his intellectual legacy is enormous and perennial. The world is different today because he was with us for many years."