Edward Bradford Titchener, answered from the texts and cited to the page.
Every science rests upon observation, and the observation proper to psychology is introspection — the inspection by a trained observer of his own conscious experience under controlled conditions.123 This is not the casual "looking within" of the amateur, which yields nothing but confusion; it is a disciplined skill, learned as one learns to use a microscope, in which the observer holds an experience steady and describes its sensory contents and their attributes exactly as they are given.
Above all he must avoid the stimulus error — the fatal mistake of reporting the object, the meaning, the thing perceived, in place of the bare conscious content itself. To say "I see a book" is not psychology; to describe the brownish rectangular patch of shifting shades that is the actual datum of consciousness — that is psychology. Introspection so trained and so guarded is as objective as any laboratory observation: the observer's report is the fact, and where the conditions are controlled and the training sound, the reports of different observers agree. There is no other door to the mind, and a psychology that shuts it shuts itself out of its own subject.45
The observation may be repeated; it may be isolated, — you may be seated in a dark and silent room, free from disturbances; and it may be varied, — different words may be called out, the word may be flashed upon a screen instead of spoken, etc. Here, however, there seems Vo be a difference between introspection and inspection.A Text-book of Psychology, p. 22
The first objection is unanswerable. But although we can never apply the introspective method to any consciousness except our own, we can arrange matters so that other individuals may be brought forward as witnesses to the facts which we ourselves have observed. This end is attained by the employment of the method under experimental conditions.An Outline of Psychology, pp. 35–36
For our own mind is our only means of interpreting the mind of another organism; we cannot imagine processes in another mind that we do not find in our own. Experimental introspection is thus our one reliable method of knowing ourselves; it is the sole gateway to psychology. Psychology is a very old science: we have a complete treatise from the hand of Aristotle (b.c. 384-322).A Primer of Psychology
I cannot enjoy and examine my enjoyment at one and the same time. Psychological introspection, however, does not consist in the effort to follow up a process during its course.An Outline of Psychology, pp. 33–35
We try, further, to keep the rest of the cortex steady (i) by shutting out other stimuli. Thus we work in a darkened room, and flash the word on a screen, all else remaining dark. And (2) we do all we can to preserve an equable frame of mind; knowing that if the thoughts are allowed to wander, new processes will be arising in various parts of the cortex, and the equilibrium of the brain will have been upset.A Primer of Psychology
Introspection was structuralism's method and became its undoing. The demand for trained introspective report set real standards of controlled observation, but the method could not settle its own disputes: the imageless-thought controversy pitted Titchener's Cornell against the Würzburg laboratory in a disagreement that introspection itself could not resolve, exposing its dependence on training, expectation, and theory. Watson's behaviorism (1913) rejected introspection as private and unverifiable and won the field. Yet the story does not end there: modern consciousness science has partially rehabilitated first-person and introspective report — with methodological safeguards and alongside objective measures — so that Titchener's central problem (how to make the description of experience scientific) is live again, even as his claim that introspection is sufficient and self-sufficient is rejected.