The aforementioned ways of capturing the prime facts of psychology are all defective; no true science can content itself with such inadequate material. So Herbart turns to his method of relations. Only insofar as a particular representation may be picked out of the “whole mass of inner perceptions” (PsW: 33), in order to serve as a known relatum, standing in relation to some other, as yet unknown relatum—i.e., its condition of possibility—only to that extent does it function as a principle of psychology (PsW: 15).[8]  Psychology, if it is to be scientific, must supplement and complete the empirical material (as such but a wild flux) by way of a principle of determinate stability: psychology aims to “prove—by means of what perception cannot attain—the nexus of what has let itself be perceived” (PsW: 32; cf. PsW: 131; Weiss 1928: 72–3). But thinking the empirically given with the aid of something hidden is only possible through metaphysics, which explains all being (Seiendes) (Hartenstein 1850: viii–ix; cf.  esp. PsW: 32–3). Only in this way can empirical psychic facts be brought under a universal set of laws and overcome their isolation from physical phenomena (Weiss 1928: 73). Universal metaphysical principles, valid across the psychophysical schism, provide the framework within which the psychic facts can be “completed” and become proper objects of scientific understanding.