Wilhelm Wundt, answered from the texts and cited to the page.
The objection I have heard most often is that experimental psychology abolishes introspection — that in submitting consciousness to laboratory conditions, it destroys the very thing it set out to study. This rests on a confusion between two entirely different procedures that happen to share a name. The loose self-observation of the older philosophy — the practice of turning attention inward and reporting whatever floats through the mind without further assistance — is not observation in any scientific sense.
Its representatives, left to this subjective method alone, hold radically divergent opinions on hardly a single question of fact, and the reason is plain: an inaccurate inner perception, unsupported by any external control, is inevitably exposed to the grossest self-deception.1 What I introduced at Leipzig is something altogether different.
The experimental procedure does not banish introspection; it substitutes for that professing self-observation a true and reliable introspection, by bringing consciousness under accurately adjustable objective conditions.1 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.
There is a further reason, peculiar to psychology and not applicable to the natural sciences, that makes experiment indispensable rather than merely useful. In studying nature we deliberately abstract from the perceiving subject; in psychology that abstraction is forbidden by our fundamental principles. And the very intention to observe — which is the necessary condition of all observation — modifies essentially the rise and progress of psychical processes, or suppresses them entirely.2
Pure introspection, left to itself, thus undermines its own object. Experiment resolves this difficulty: a sensation arises under the most favorable conditions for observation when it is caused by an external sense-stimulus, a tone-sensation from a tone-vibration, a light-sensation from an external light-impression, and if we wish to study how an idea is formed, we can choose and vary those external causes at will.3
The reaction-experiment shows the method at work most clearly. A signal concentrates the attention; a stimulus acts on a sense-organ; the observer executes a prepared movement at the moment of perception. The psychological conditions correspond to those of a simple volition, and by deliberately varying the conditions the experimenter can influence the succession of psychical processes in a systematic manner and follow it with exactness through introspection.4
The measurement of the rate of psychical processes is a secondary yield; the primary significance is that each experiment includes a volition and makes its inner course accessible to controlled observation.5 One further boundary must be kept clear. The name "experimental psychology" has been applied in France chiefly to hypnotism and suggestion, and this usage narrows the definition in a wholly unacceptable way.
If we call any operation upon consciousness that changes its contents a psychological experiment, then narcosis and the purposeful interference with dream-consciousness fall under the same heading — and that cannot be right.6 The value of the psychological experiment lies precisely in the fact that it makes exact introspection possible; most hypnotic modifications of consciousness fail this test entirely, and their results, however interesting in isolation, have no secure place in the psychological system.6
So: introspection, properly understood, is not the armchair report of whatever drifts through a private mind. It is the inner observation of a consciousness whose conditions have been set, controlled, and varied from without. That is what makes it scientific, and that is what the experimental method at Leipzig was built to achieve.
The only form of introspection which experimental psychology seeks to banish from the science is that professing self-observation which thinks it can arrive directly, without further assistance, at an exact characterisation of mental facts... The aim of the experimental procedure is to substitute for this subjective method, whose sole resource is an inaccurate inner perception, a true and reliable introspection, and to this end it brings consciousness under accurately adjustable objective conditions.Principles of Physiological Psychology Vol I
the very intention to observe, which is a necessary condition of all observation, modifies essentially the rise and progress of psychical processes. Observation of nature is not disturbed by this intention on the part of the observer, because here we purposely abstract from the state of the subject.Outlines of Psychology
A sensation arises in us under the most favorable conditions for observation when it is caused by an external sense-stimulus, as, for example, a tone-sensation from an external tone-vibration, or a light-sensation from an external light-impression.Outlines of Psychology
A short interval (2–3 sec.) after a signal that serves to concentrate the attention, an external stimulus is allowed to act on some sense-organ. At the moment when the stimulus is perceived, a movement that has been determined upon and prepared before... is executed.Outlines of Psychology
The primary significance of the experiments, however, consists in the fact that each one includes a volition and that it is therefore possible, in this way, by means of introspection to follow with exactness the succession of psychical processes in such a volition, and at the same time, by the deliberate variation of the conditions, to influence this succession in a systematic manner.Outlines of Psychology
It has become customary, more especially in France, to employ the name principally, if not exclusively, for experiments upon hypnotism and suggestion... if the principal value of the psychological experiment lies in the fact that it makes an exact introspection possible, very few of these modifications of consciousness can be termed true psychological experiments.Principles of Physiological Psychology Vol I