  One might think that … we can assign probabilities to the various possible outcomes. Nothing in the Newtonian physics requires us to assign the probabilities, but we might choose to try to add them for our own conceptual comfort. It can be done as far as the direction of the spontaneous motion is concerned. The symmetry of the surface about the apex makes it quite natural for us to add a probability distribution that assigns equal probability to all directions. The complication is that there is no comparable way for us to assign probabilities for the time of the spontaneous excitation that respect the physical symmetries of solutions. Those solutions treat all candidate excitation times \(T\) equally. A probability distribution that tries to make each candidate time equally likely cannot be proper—that is, it cannot assign unit probability to the union of all disjoint  outcomes.[22]  Or one that is proper can only be defined by inventing extra physical properties, not given by the physical description of the dome and mass, Newton’s laws and the laws of gravitation, and grafting them unnaturally onto the physical system. (Norton 2003: 9–10)