The four key concepts in James' book are: stream of consciousness (his 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).

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 humans while the functions of the cerebrum itself become increasingly flexible and less localized as one moves along the same continuum.

Human habits are constantly formed to achieve certain results because of one's strong feelings of wanting or wishing for something.

James emphasized the importance and power of human habit and proceeded to draw a conclusion.

Impulses which, when observed out of their greater context, may have appeared just as automatic as the most basic of animal instincts.

Philosopher Helmut R. Wagner writes that most of the book's contents are now outdated, but that it still contains insights of interest.

The philosopher Edmund Husserl engages specifically with William James's work in many areas.