 The PSR is nearly as old as philosophy itself. Anaximander, one of the earliest of the pre-Socratics, is usually credited—on the basis of Aristotle’s de Caelo, (b12 295b10–16)—with being the first to make use of it. Anaximander argues, we are told, that the Earth remains stationary in space because it is indifferent between motions in any direction. This indifference means that there is no reason why it should move in one direction rather than another. Since he concludes from this that it does not move, we can assume that Anaximander believes that motion in the absence of a reason is impossible.