Psychological introspection goes hand in hand with the methods of experimental physiology. If one wants to put the main emphasis on the characteristic of the method, our science, experimental psychology, is to be distinguished from the ordinary mental philosophy [Seelenlehre], based purely on introspection.

Grundzüge der physiologischen Psychologie [Principles of Physiological Psychology] (1874), 2–3. Trans. K. Danziger, Constructing the Subject: Historical Origins of Psychological Research (1990)