"[W]e find from the theory that if we disturb the electron, we may cause a transition from a positive-energy state of motion to a negative-energy one, so that, even if.. all.. electrons in the world.. started.. in positive-energy states, after a time some... would be in negative-energy states. ...[B]ehaviour of these states in an electromagnetic field shows that they correspond to the motion of an electron with a positive charge ...a . One might... assume that electrons in negative-energy states are just positrons, but ...observed positrons ...do not have negative energies."