Born in New York City, he graduated in medicine from Harvard, where he taught comparative anatomy from 1872, t hen philosophy from 1882. Professor from 1885, he changed his professorial title in 1889 from philosophy to psychology. A pioneer psychologist, he influenced both branches of his subject, the behaviouristic and the introspective. In his Principles of Psychology (1890) he places psychology firmly on a physiological basis and represents the mind as an instrument for coping with the world.
He transformed the complex and obscurely expressed opinions of the philosopher Charles Sanders Peirce into the popular philosophy of pragmatism and described himself as a 'radical empiricist', maintaining that metaphysical disputes can be resolved or dissolved by examining the practical consequences of competing theories. He expounded these ideas most famously in The Will to Believe (1907) and Pragmatism (1907), and he treated ethics and religion in the same practical, non-dogmatic way, as in The Varieties of Religious Experience (1902) and The Meaning of Truth (1909). James and Henri Bergson were responsible for the formulation of the concept of 'stream of consciousness', a term which James himself coined. In his sympathetic Varieties of Religious Experience (1902) he showed himself to be a master of literary style, and simultaneously encouraged his readers, in an age of loss of faith, not to be afraid of religious and mystical experiences. He exercised a great influence both on politicians (who, like Mussolini, often misunderstood him) and on writers, such as his pupil Gertrude Stein. He also helped to found the American Society for Psychical Research. He was the elder brother of Henry James.