Harvard psychologist and philosopher, brother of the novelist Henry James, and — with Dewey and C.S. Peirce — a founding figure of American pragmatism. James trained as a physician, suffered a long young-adult depression that colored his mature thinking about will and belief, and gave American philosophy its first distinctive voice — worldly, empirical, pluralist, temperamentally generous toward religious experience but ruthlessly suspicious of philosophical abstraction unmoored from consequences.
His Principles of Psychology (1890) is the 19th century's great textbook in what was still an emerging science — twelve hundred pages containing, among other things, the concept of the "stream of consciousness" (which his brother would turn into a literary technique), a functional account of habit, and a naturalistic theory of emotion. His Varieties of Religious Experience (1902), the Gifford Lectures, took religious experience seriously on its own terms as a subject of psychological investigation — attending to the conversion stories of ordinary believers rather than dismissing them as delusions or reducing them to theology. The book remains the best single entry point to a certain American religious sensibility.
His philosophical books — Pragmatism (1907), A Pluralistic Universe (1909), the posthumous Essays in Radical Empiricism — developed the position for which he is most remembered. Truth, on the pragmatist account, is not a static correspondence with a timeless order but what works, what makes a genuine difference, what "cash value" a belief has in actual experience. The "will to believe" essay (1896) argued, against evidentialist ethics, that on genuinely undecidable questions (including some religious ones) we are entitled to believe ahead of evidence when belief is itself the condition of acting well.
His influence on 20th-century philosophy waned under the logical-positivist tide, then recovered with Rorty, Putnam, and the pragmatist revival. His readability never flagged.