 Psychology, as a part of philosophy, had already several times changed the way it defined its object: as “soul”, “mental substance”, “mind”, etc. By the late eighteenth and early nineteenth centuries, many regarded psychology to be the account of consciousness or “inner experience”, distinct from the natural scientific accounts of external, sensible reality. After having dealt the coup de grâce to the speculative, rational, a priori psychology of the soul epitomized by Christian Wolff, however, Kant tried to cut off any retreat into the empirical study of consciousness, as well. In the Metaphysical Foundations of Natural Science, he argued that empirical psychology cannot be an exact science because the phenomena it seeks to explain are not mathematically expressible (Kitcher 1990: 11). Moreover, it can never become an experimental science “because it is not possible to isolate different thoughts” (Kitcher 1990: 11). Finally, and most fatally, the only access to the phenomena of inner experience, introspection, ipso facto alters those phenomena: if I try, by introspection, to study what it’s like to be tristful, the phenomena of my sadness are now something different, namely, phenomena of my sadness-being-studied-by-me (Kitcher 1990: 11). Thus psychologists found their object declared beyond the limit of possible investigation and their methods vain. While such arguments did not persuade all of Kant’s successors of the hopelessness of their enterprise, their attempts were unpromising. On the one hand, the German Idealists’ fanciful speculation about Geist collapsed upon itself. On the other hand, the efforts of  J.F. Herbart  to devise a mathematical mental mechanics suggested a possible way forward although in the end it proved equally fruitless. Thus, for those mid-nineteenth-century enthusiasts of mental phenomena, the future of a genuine psychology seemed blocked.