In 1854, at the Crystal Palace exhibition in New York, Elisha Otis rode a platform up an open shaft, and when it was well off the ground he had the rope that held it cut. The platform stayed where it was. Lifts already existed. What Otis had built was a way for them not to fall: a steel wagon spring above the platform, kept bent by the pull of the hoisting rope, with ratchet teeth along the guide rails on either side. While the rope pulls, the spring stays bent and clear of the teeth. When the rope goes slack, the spring snaps straight and catches them. Nobody has to notice that anything has gone wrong, and nothing has to switch on. The part that stops the lift is pushing to stop it the whole time, and the rope is all that holds it back. The machine doesn’t need to be told to stop. It needs to be allowed, every second it moves, to keep going.