Extending his mathematical psychology by analogy to the theory of motion, Herbart complements his psychic statics (PsW: 139), or doctrine of representational equilibrium, with a psychic mechanics (PsW: 208), or doctrine of movement of these “mental bodies” over time (PsW: 137; cf. Boudewijnse, et al. 1999: 163). It is to these that we now turn.