 As we have seen, Wundt was concerned not only with expanding the set of known psychological facts, but also with interpreting them within an appropriate explanatory framework. Of course, the necessity of establishing such a closed framework distinct from physiology amounted to distinguishing psychological causality from physical causality in general, and hence psychology from the natural sciences altogether. But psychology has to be defined against two other areas of “scientific” (wissenschaftlich) inquiry; first, in its völkerpsychologisch dimension, against the Geisteswissenschaften or “human sciences”, and second, against the non-psychological domains of philosophy. As these relationships are laid out below, it must always be remembered that although these four areas—psychology, philosophy, natural science, human science—are irreducible, this irreducibility is not a metaphysical or ontological one, but merely one of explanatory function (and commensurate methodology). They do not have distinct objects, but again merely represent ways of describing irreducible perspectives upon the same object, namely experience. Wundt writes: