The Principles of Psychology is a monumental text in the history of psychology, written by William James and published in 1890. William James (1842-1910) was an American philosopher and psychologist who was also trained as a physician.

There were four methods in James' psychology: analysis (the logical criticism of precursor and contemporary views of the mind), introspection (the psychologist's study of his own states of mind), experiment (in hypnosis or neurology), and comparison (the use of statistical means to distinguish norms from anomalies).

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.