 The PPP has caused a great deal of confusion in the secondary literature, which persists in characterizing it as a  metaphysical[42]  doctrine somehow derived from Leibniz (e.g., Wellek 1967: 350; Thompson and Robinson 1979: 412) or Spinoza (cf. L I: 77). Wundt however is crystal-clear that the PPP is not a metaphysical “hypothesis”. It is merely an admittedly misleading name for an “empirical postulate” necessary to explain the phenomenal “fact” of consciousness of which we are immediately aware (Wundt 1911a: 22; cf. esp. 28). By denying any metaphysical interpretation of his principle, Wundt insists that the “physical” and the “psychic” do not name two ontologically distinct realms whose events unfold on separate yet parallel causal tracks. He is therefore not an epiphenomenalist, as some commentators have claimed. Rather, the “physical” and “psychic” name two mutually irreducible perspectives from which one and the same world or Being (Sein) may be observed: “nothing occurs in our consciousness that does not find its sensible foundation in certain physical processes”, he writes, and all psychological acts (association, apperception, willing) “are accompanied by physiological nerve-actions” (PP II: 644). In distinguishing the empirical from the metaphysical PPP, Wundt contrasts his own view against Spinoza’s, which, according to Wundt, makes the realm of material substance exist separately from, though parallel to that of mental substance (Wundt 1911a: 22, 44–5; cf. esp. Wundt 1911a: 143, ff.).