William James, working across physiology, psychology, and philosophy, became the leading founder of American pragmatism. In "Pragmatism" (1907) he argued that the meaning and truth of an idea should be assessed by its practical consequences for experience rather than by correspondence to an abstract "really real"; in "The Varieties of Religious Experience" (1902) he examined religious belief empirically through its psychological forms and practical fruits; and in "The Principles of Psychology" (1890) he introduced the "stream of consciousness" and helped found functionalist psychology.
Intellectual setting
James taught at Harvard from 1873 until 1907, moving between physiology, psychology, and philosophy as those disciplines were institutionally separating in American universities; he wrote against the background of Darwinian biology, the new experimental psychology developing in Germany under Wilhelm Wundt, and a broader American reaction against both rigid rationalist idealism and reductive scientific materialism.
Key ideas
The pragmatic theory of truth and meaning
James argued that the meaning of a concept lies in its practical effects and that a belief counts as true so far as it "works" - proves useful and satisfactorily fits with the rest of experience - turning abstract metaphysical disputes into questions about the practical difference an idea makes.
The stream of consciousness and radical empiricism
In "The Principles of Psychology" James described conscious experience as a continuous "stream of thought" rather than a sequence of discrete sensations, and later developed "radical empiricism," the view that the relations between experiences are themselves directly given in experience, anticipating later phenomenology and process philosophy.
Legacy
James's pragmatism was extended by John Dewey and later reworked by philosophers including Richard Rorty and Hilary Putnam, making it one of the central traditions in American philosophy; "The Varieties of Religious Experience" continues to shape the psychology of religion, and his psychological writings influenced functionalism, Edmund Husserl's phenomenology, and figures from Bertrand Russell to the pragmatist revival of the late twentieth century.