Wilhelm Wundt, answered from the texts and cited to the page.
Psychology is the science of immediate experience — of the world not as it is abstracted from the observer, which is the physicist's concern, but as it is actually given to the observer who lives it.1 There is only one way to study immediate experience, and that is to observe it; but the loose self-inspection of the old philosophers, the armchair report of whatever floats through the mind, is not observation in any scientific sense — it changes what it looks at, it cannot be repeated, it cannot be checked.
What I introduced at Leipzig is something else: experimental introspection, in which the experimenter controls and varies the stimulus, the moment of observation is fixed, the conditions are repeatable, and the trained observer reports the immediate contents of a consciousness set going under known conditions.23 This makes the inner observation as objective as the outer, because it is bound to a measurable stimulus and a repeatable procedure.
And it has its limits, which I have always marked: it reaches the simple processes — sensation, the time of a reaction, the dimensions of feeling — where the condition can be controlled; the complex reasoning of the higher mind slips the control and cannot be got at this way. Within its proper scope it is a genuine method, and it is the method with which psychology became a science.45
The recognition of the immediate reality of psychological experience renders impossible any attempt to derive the particular components of psychical phenomena from processes specifically different from the experiences themselves.Outlines of Psychology, pp. 46–47
Leipzig, March 1901 and January 1905. W. Wundt. CONTENTS. INTRODUCTION. I. Problem of Psychology I. Older definitions. 2. Psychology as the science of immediate experience. 3. Relation to the mental and to the natural sciences. 3a. Knowledge as gained through the natural sciences mediate and conceptual, that gained in psychology immediate and perceptual. 2. General Forms of Psychology 6 I.Outlines of Psychology, pp. 18–20
But conscious contents are at the opposite pole from permanent objects; they are processes, fleeting occurrences, in continual flux and change. In their case, therefore, the experimental method is of cardinal importance; it and it alone makes a scientific introspection possible.Principles of Physiological Psychology Vol I, p. 5
When the scientific investigator is inquiring into the causes of a phenomenon, he does not confine himself to the investigation of things as they are given in ordinary perception. That would never take him to his goal, though he had at his command the experiences of all time.Lectures on Human and Animal Psychology, pp. 22–23
Its aim is now more general; it attempts, by arbitrary modification of consciousness, to arrive at a causal analysis of mental processes. Fechner's determinations are also affected, to some extent, by his conception of psychophysics as a specific science of the ' interactions of mind and body. But, in saying this, we do not lessen the magnitude of his achievement.Principles of Physiological Psychology Vol I, p. 7
The founding methodological claim of experimental psychology and the one most decisively rejected by the century that followed. Behaviorism, from Watson forward, cast out introspection entirely as private, unverifiable, and unscientific, and made its elimination the mark of a mature psychology. The rejection, however, blurred a distinction Wundt insisted on: his experimental introspection was narrow, stimulus-bound, and confined to the simple processes, quite unlike the elaborate qualitative self-analysis of Titchener's structuralism or the free self-report the behaviorists attacked. The imageless-thought controversy (the Würzburg school's claim to introspect thought without sensory content) exposed the method's difficulties for the higher processes — difficulties Wundt met by holding that such matters lay outside proper experimental introspection. Historians (Danziger) have recovered the actual, restricted Wundtian method; the general question of whether trained self-observation can be scientific evidence remains contested wherever consciousness is studied.