The severe restrictions which Wundt placed on introspection also manifest themselves in the types of judgment that his experimental subjects were required to make. In accordance with the precept that internal perception can only become observation insofar as it is linked to controllable external stimuli, the introspective reports from his laboratory are very largely limited to judgments of size, intensity, and duration of physical stimuli, supplemented at times by judgments of their simultaneity and succession.

p. 247 - "The history of introspection reconsidered." 1980