structural:expansion, information:extraction
Psychophysical parallelism

Influenced by Leibniz, Wundt introduced the term psychophysical parallelism as follows: "… wherever there are regular relationships between mental and physical phenomena the two are neither identical nor convertible into one another because they are per se incomparable; but they are associated with one another in the way that certain mental processes regularly correspond to certain physical processes or, figuratively expressed, run 'parallel to one another'." Although the inner experience is based on the functions of the brain there are no physical causes for mental changes.

Leibniz wrote: "Souls act according to the laws of final causes, through aspirations, ends and means. Bodies act according to the laws of efficient causes, i.e. the laws of motion. And these two realms, that of efficient causes and that of final causes, harmonize with one another." (Monadology, Paragraph 79).

Wundt follows Leibniz and differentiates between a physical causality (natural causality of neurophysiology) and a mental (psychic) causality of the consciousness process. Both causalities, however, are not opposites in a dualistic metaphysical sense, but depend on the standpoint. Causal explanations in psychology must be content to seek the effects of the antecedent causes without being able to derive exact predictions. Using the example of volitional acts, Wundt describes possible inversion in considering cause and effect, ends and means, and explains how causal and teleological explanations can complement one another to establish a co-ordinated consideration.

Wundt's position differed from contemporary authors who also favoured parallelism. Instead of being content with the postulate of parallelism, he developed his principles of mental causality in contrast to the natural causality of neurophysiology, and a corresponding methodology. There are two fundamentally different approaches of the postulated psychophysical unit, not just two points-of-view in the sense of Gustav Theodor Fechner's identity hypothesis. Psychological and physiological statements exist in two categorically different reference systems; the important categories are to be emphasised in order to prevent category mistakes as discussed by Nicolai Hartmann. In this regard, Wundt created the first genuine epistemology and methodology of empirical psychology (the term philosophy of science did not yet exist).