One day, René Descartes was walking down a street and turned a corner. Lost in deep philosophical thought, he ran into another man, knocking him down to the ground. As he made his embarrassed apologies and helped the other man up, he was surprised to find the person in front of him looked a lot like himself. The stranger also stared back, obviously thinking the same thing. Descartes introduced himself to find the other man had the same name. They went to a nearby bistro and had a long talk. There, they developed the concept of Cartesian duality that allowed them to confuse waiters by each ordering the same thing and being charged for only one serving.