The second volume of William James's seminal treatise on psychology delves into the complex processes by which human beings cognize the material world, beginning with an intricate analysis of sensation and perception. James rigorously distinguishes between pure sensation—which he treats as a rare abstraction mostly confined to infancy—and perception, wherein immediate sense-impressions are supplemented by revived sensations and associative paths from past experience. He argues that perception is not an act of unconscious reasoning, as many contemporaries claimed, but a direct physiological reaction shaped by brain habits and probable outcomes. A large portion of the text is dedicated to examining optical illusions, the relativity of knowledge, and the physiological mechanics of color contrast, wherein James contrasts psychological theories of judgment with Hering's physiological explanations of retinal changes.
Moving from outer perception to imagination, James explores the vast individual differences in mental imagery, drawing heavily on Galton's statistical inquiries. He categorizes minds into visual, auditory, and motor types, demonstrating how each individual relies on distinct sensory 'thought-stuff' during memory and reasoning. The neural basis of imagination is evaluated through the lens of cortical activity, noting the sharp subjective divide between weak imaginative ideas and vivid sensational processes. Hallucinations and illusions are then systematically examined, interpreted not merely as projected mental images, but as extreme states of the perception process where normal cortical inhibition fails and internal tension or faint peripheral stimulation explodes into full sensational intensity.
The volume culminates in an extensive treatment of the will and the production of movement. James asserts that all consciousness is impulsive, and that voluntary action requires the central idea of a movement to dominate the mind, unhindered by conflicting sensations. He challenges the traditional views on free-will and determinism, arguing that psychology as a science must adopt determinism while education of the will relies on the sustained effort of attention. Throughout these final chapters, James maintains that mental states are integral, unified events rather than mechanical bundles of ideas, leaving a lasting framework for modern cognitive science and philosophy.
