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The Principles of Psychology is an 1890 book about psychology by William James, an American philosopher and psychologist who trained to be a physician before going into psychology. There are four methods from James' book: stream of consciousness (James' most famous psychological metaphor); emotion (later known as the James–Lange theory); habit (human habits are constantly formed to achieve certain results); and will (through James' personal experiences in life).

Nineteenth-century experimental results

The 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.

Stream of consciousness

Stream of consciousness is arguably James' most famous psychological metaphor.^[1] He argued that human thought can be characterized as a flowing stream, which was an innovative concept at the time due to the prior argument being that human thought was more so like a distinct chain. He also believed that humans can never experience exactly the same thought or idea more than once. In addition to this, he viewed consciousness as completely continuous.

Emotion

James introduced a new theory of emotion (later known as the James–Lange theory), which argued that an emotion is instead the consequence rather than the cause of the bodily experiences associated with its expression.^[1] In other words, a stimulus causes a physical response and an emotion follows the response. This theory has received criticism throughout the years since its introduction, but regardless, it still has its merits.

Habit

Human habits are constantly formed to achieve certain results because of ones strong feelings of wanting or wishing for something. James emphasized the importance and power of human habit and proceeded to draw a conclusion. James noted that the laws of habit formation are unbiased, habits are capable of causing either good or bad actions. And once either a good or bad habit has begun to be established, it is very difficult to change.^[1]

Will

Will is the final chapter of The Principles of Psychology, which was through James' own personal experiences in life. There was one question that troubled James during his crisis, which was whether or not free will existed.^[1] "The most essential achievement of the will,... when it is most 'voluntary', is to attend to a difficult object and hold it fast before the mind..." Effort of attention is thus the essential phenomenon of will."^[1]