 As we have seen  (Section 3.2),  for Wundt the possibility of a physiological psychology (as opposed to a purely physiological inquiry into sensation, behavior, learning, etc.) depends on the possibility of self-observation. Self-observation, in turn, is of scientific use only if the sequence of “inner” phenomena of consciousness is assumed to fall under an independent principle of psychic causality. For if it does not, then these phenomena could never be more than a chaotic muddle, of which there could be no science. Alternatively, if the “inner” phenomena could be shown to fall under the physical causality of the natural sciences, then there would be no need for a special psychological method, such as self-observation (cf. Natorp 1912). In fact, however, a system of psychic causality can be determined, Wundt argues, one that at no point is reducible to physical causality: “no connection of physical processes can ever teach us anything about the manner of connection between psychological elements” (Wundt 1894: 43, quoted in Kusch 1995: 134). This “fact”, which Wundt thinks is given in the psycho-physiological experiments described above, leads him to his so-called principle of psychophysical parallelism (PPP).