Principles of Psychology
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There were four methods in James' psychology: analysis (i.e. the logical criticism of precursor and contemporary views of the mind), introspection (i.e. the psychologist's study of his own states of mind), experiment (e.g. in hypnosis or neurology), and comparison (the use of statistical means to distinguish norms from anomalies).
Analytical arguments of the Principles 
There were five chief targets of the critical/analytical arguments of the volume: innatism (typified by Immanuel Kant); associationism (by Jeremy Bentham); materialism (by Herbert Spencer); spiritualism (by scholastic theology); and metaphysical idealism (by Georg Wilhelm Friedrich Hegel).
The perception of time was a very hotly contested field in the psychology of James' day, and gave him an opportunity to explain the difficulty with innatism, which posits time as an infinite necessary continuum. This is a view that leads to unnecessary paradoxes and defies experience. What we experience, rather, are immediate memories and expectations, in a "specious present" of a few second's duration, and all longer spans of time are extrapolations from that.
But just as innatism gives the mind too much credit for time and space, associationism gives it too little credit for art and creativity in general. It treats ideas as bumping into each other and forming broader patterns, even in the end novels and architectural blueprints, in much the same way that atoms bump into one another to form molecules. In this way, it bans the fact of intellectual power.
In James' day, the salient effort to give a thoroughly materialistic account of mind was that of Herbert Spencer. James demonstrates the great confusion inherent in this account. On the one hand, Spencer denied that material facts can ever give rise to feelings, in statements that would seem to commit him to dualism. "Can the oscillations of a molecule," Spencer asked rhetorically, "be represented side by side with a nervous shock, and the two be recognized as one? No effort enables us to assimilate them."
But then Spencer proceeded to attempt to assimilate the two. Later, looking back on his discussion of the point, Spencer wrote how "in tracing up the increase we found ourselves passing without break from the phenomena of bodily life to the phenomena of mental life." The way in which Spencer got from the former declaration to the latter involved what James called the mind-dust theory, and the self-compounding of mental facts, reducible to (and subject to the same objections as) associationism.
Scholasticism is "popular philosophy made systematic." In psychology, it is the theory that mental events are to be attributed to a special intangible substance known as the soul. James conceded that it might be accurate, but said that "it is at all events needless for expressing the actual and subjective phenomena of consciousness as they appear." The phenomena can be expressed more economically with the "supposition of a stream of thoughts" each cognitive of its precursors and claiming them as its own.
Nineteenth century experimental results 
The opening of Principles, after introductory material, presented what was known at the time of writing about the localization of functions in the brain—how each sense seemed to have a neural center to which it made report, and how varied bodily motions have their sources in still other centers.
The particular hypotheses and observations on which James relied are of course very much dated. But the broadest conclusion to which his material leads is still valid: that the functions of the "lower centers" (beneath the cerebrum) become increasingly specialized as one moves from reptiles, through ever more intelligent mammals, to humans, while the functions of the cerebrum itself become increasingly flexible, less localized or specialized, as one moves along the same continuum.
James discussed experiments on illusions, too (optical, auditory, etc.), and offered a physiological explanation for many of them, that "the brain reacts by paths which previous experiences have worn, and makes us usually perceive the probable thing, i.e. the thing by which on previous occasions the reaction was most frequently aroused." Illusions are thus a special case of the phenomenon of habit.
Principles is an important source for the history of psychology in the 19th century. In Chapter 7, James suggests that Weber, Fechner, Vierordt, and Wundt might be the greatest experimental psychophysiologists of the 19th century.
Consequences of comparisons 
In the use of the comparative method, James wrote, "instincts of animals are ransacked to throw light on our own...." By this light, James dismisses the platitude that "man differs from lower creatures by the almost total absence of instincts." There is no such absence, so the difference must be found elsewhere.
Humanity has, indeed, a far greater variety of inborn unreasoned impulses than any other animal, and any one of those impulses taken by itself is as much an "instinct" as any impulse possessed by a chicken. But in humans, instincts never operate by themselves for long. They soon give rise to memories and are mixed with expectations of consequences so that gradually, as a child grows to adulthood, the instincts are brought within the bounds of a single, unified, responsible personality.
Publication data 
- The Principles of Psychology (1890), with introduction by George A. Miller, Harvard University Press, 1983 paperback, ISBN 0-674-70625-0 (combined edition, 1328 pages)
- The Principles of Psychology, (1890), Dover Publications, 1950 paperback, vol. one, 696 p. ISBN 0-486-20381-6, vol. two, 708 p. ISBN 0-486-20382-4