The work opens by defining psychology as the science of mental life, encompassing both mental phenomena and their physical conditions. James establishes that the brain is the immediate bodily condition of mental operations, rejecting both purely spiritualistic and associationist metaphysical theories in favor of a strictly positivistic and cerebralist point of view. He examines the functions of different brain regions, using evidence from comparative anatomy, electrical stimulation, cortical ablations, and pathological clinical cases such as aphasia to map motor and sensory localizations.
Particular attention is given to the motor zone surrounding the fissure of Rolando, which governs voluntary movements, and to specialized cortical areas connected with sight, hearing, smell, and touch. The author analyzes the phenomenon of brain plasticity and reaction-time experiments, demonstrating how nervous events can be measured and how physical stimuli summate within neural tracts. These physiological mechanisms form the material substrate for mental activity and behavior.
The central doctrine of habit is explored as a physical phenomenon rooted in the plasticity of organic matter. James explains how the nervous system grows to the modes in which it is habitually exercised, resulting in simplified movements, increased accuracy, reduced fatigue, and diminished conscious attention during complex actions. Habit acts as the great conservative fly-wheel of society, determining personal character and professional tendencies once the nervous system has set.
The final portions of the work address theoretical problems concerning brain activity and the relation between mind and body, including the automaton-theory. James evaluates the physiological evidence regarding cerebral blood-supply, temperature changes, and the distribution of consciousness in lower animals compared to humans. Throughout the treatise, the emphasis remains fixed on the empirical correlation between mental manifestations and their underlying cerebral conditions, providing a comprehensive foundation for modern scientific psychology.
