The book begins by establishing psychology as a natural science closely allied with physiology, positing that all mental states are inextricably bound to neural changes in the brain. James outlines the divisions of psychology, focusing first on sensations derived from the external world through sensory organs such as the eye and the ear. He examines the specific energies of various brain regions, the distinctions between sensation and perception, and Weber's law regarding the intensity of stimuli. Moving through touch, the temperature sense, muscular feeling, and pain, the author analyzes the sensations of motion, both across sensory surfaces and through translation in space via the semicircular canals.
A detailed anatomical and functional overview of the brain follows, illustrating how cerebral structures correspond to sensory and motor capacities. The text further investigates brain functions, motor localization, and phenomena like aphasia, where specific cortical injuries disrupt language and comprehension. Turning to general neural activity, James discusses reaction-time experiments, cerebral blood-supply, and the physical basis of habit, demonstrating how plasticity in the nervous system shapes human behavior, character, and society. The volume culminates in an introspective analysis of the stream of consciousness, detailing its continuity, changing nature, selective attention, and the complex structure of the personal self.
