Dirac’s original expression actually involved the rate of change of the acceleration vector on the right-hand side. The resulting equation gives rise to the well-known problem of runaway solutions. To avoid such unphysical behaviour I have submitted Dirac’s equation to a reduction-of-order procedure whereby n da /dt is replaced with -1 n m dfext/dt. This procedure is explained and justified, for example, in [47Jump To The Next Citation Point26Jump To The Next Citation Point].