 Each of these paths has its drawbacks.  (1) Intentional or self-conscious introspection interferes in the natural operation of psychic processes (LPs [1850]: 8). Even if introspection happens incidentally, Herbart points out that the observer always already introduces some prior knowledge of himself that colors the resulting observation (SW V: 192; cf. [1840]: 36).  (2) Interpretation of the products of my own activity obviously cannot serve to reveal the principles, since the products are ex hypothesi already removed from those principles. Rather, the products of conscious activity are the very phenomena of which our science must seek the causes (PsW: 20–21; SW V: 194).  Finally, (3) the interpretation of others is inadequate because in addition to its inherent unreliability, even the most faithful testimony again depends on the informant’s introspection. Moreover, the psychologist’s interpretation of that testimony will require comparison with his own knowledge of his inner conditions, so that the drawbacks of introspection make themselves felt from two sides, not just one (SW V: 198).[7]