To sum up Herbart’s approach: all mental life manifests itself in constant temporal flux and continuous mutation, as “a manifold of variegated determinations in one [in Einem]”, and ultimately, as self-consciousness (LPs [1850]: 11–12; cf. PsW: 30). We must therefore begin with general metaphysical or “speculative” postulates to compensate for the obstacles to pure observation of psychic life, and so postulate the simplicity of the soul (cf. Beiser 2014: 138). Beyond this, however, psychology must avail itself of mathematics, in that 