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Nineteenth-century experimental resultsThe openings of The Principles of Psychology 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 reported and how varied bodily motions have their sources in other centers.
The particular hypotheses and observations on which James relied are now very dated, but the broadest conclusion to which his material leads is still valid, which was that the functions of the "lower centers" (beneath the cerebrum) become increasingly specialized as one moves from reptiles, through ever more intelligent mammals, to inhumans while the functions of the cerebrum itself become increasingly flexible and less localized as one moves along the same continuum.
James also discussed experiments on illusions (optical, auditory, etc.) and offered a physiological explanation for many of them, including 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.
The Principles of Psychology covered a large number of topics, but some topics stand out as being more useful and applicable than others, particularly the sections on stream of consciousness, emotion, habit, and will.