Wilhelm Wundt was a founding father of laboratory psychology and a grand visionary of psychology as a discipline -- of how it fit among the sciences, of the structure of its object (the mind), of its methods, most centrally introspection -- and also an author so vastly prolific that most of his work remains untranslated despite his importance. Among those untranslated works is his essay "Selbstbeobachtung und innere Wahrnehmung" [Self-Observation and Inner Perception"] (1888), with which I've been struggling. The essay is key to Wundt's view of "introspection" -- the usual English translation of the German Selbstbeobachtung -- since here he contrasts it with the seemingly related process of "inner perception". And unfortunately, the secondary sources are all over the map on this. I can find no good treatments.
To understand Wundt's distinction, it helps to know two bits of historical context. One is August Comte's influential criticism of the introspective method of psychology:
But as for observing in the same way intellectual phenomena at the time of their actual presence, that is a manifest impossibility. The thinker cannot divide himself into two, of whom one reasons whilst the other observes him reason. The organ observed and the organ observing being, in this case, identical, how could observation take place? The pretended method is then radically null and void (1830, using James's translation of 1890/1981, p. 188).The other is Franz Brentano's (1874/1973) distinction between "inner observation" [innere Beobachtung] and "inner perception" [innere Wahrnehmung]. Brentano asserts that inner observation involves attending to conscious psychological processes as they transpire. This, he says with Comte, is impossible, or at least fails as a psychological method, because the act of attending to the process inevitably destroys or at least objectionably alters the target process. In "inner perception", in contrast, psychological processes are noticed while one's attention is dedicated to something else. They are noticed only "incidentally" [nebenbei], and thus undisturbed.
Wundt agrees with Brentano and Comte that observation necessarily involves attention and so normally interferes with the process to be observed, if that process is an inner, psychological one. Contra Brentano however, Wundt does not envision scientific knowledge of mental processes arising without attention of some sort, including planful and controlled variation -- attentive planned exploration, if not of the process as it occurs, then at least to a reproduction of that process as a "memory image" [Erinnerungsbild]. No science by sideways glances for Wundt. The psychological method of "inner perception" is, for Wundt, the method of holding and attentively manipulating a memory image of a psychological process. This method, he thinks, has two crucial shortcomings: First, one can only work with what one remembers of the process in question -- the manipulation of a memory-image cannot discover new elements. And second, new elements may be unintentionally introduced through association -- one might confuse one's memory of a process with one's memory of another associated process or object.
Therefore, Wundt suggests, the science of psychology must depend upon the attentive observation of mental processes as they occur. He argues that those who think attention necessarily distorts the target mental process are too pessimistic. A subclass of mental processes is relatively undisturbed by attentive observation -- specifically the basic mental processes, especially of perception. The experience of seeing red is more or less the same, Wundt suggests, whether or not one is aware of the psychological fact that one is experiencing redness. Wundt also thinks the basic processes of memory, emotion, and volition are largely undisturbed by introspective attention. These alone, he thinks, can be studied by introspective psychology. More complicated processes, in contrast, must be studied non-introspectively -- through the obsevation of language, history, culture, and human and animal development, for example.
Wundt's students tended to disregard his admonition to restrict introspective observation to such basic processes. E.B. Titchener, for example, held that practiced introspectors could observe even their "higher" cognitive processes without disturbing them. Arguably, the eventual fall of introspective psychology in favor of behaviorism (focusing only on outward stimuli and behavioral response, nothing "inner" at all) was hastened by the ambitious attempts of Wundt's students to extend introspective method to such higher cognitive processes, about which methodological and substantive disputes proved intractable.