Although he is credited with creating the first laboratory in the United States for the scientific study of psychology, he was really more interested in observing and speculating on broad issues of philosophy and religion, as evidenced by his book "The Will to Believe and Other Essays in Popular Philosophy" (1897). He tried to bring scientific analysis to the study of religion in his book "The Varieties of Religious Experience" (1902). His work is important primarily because it served as a turning point, and influenced work in a variety of disciplines, such as Einstein’s Theory of Relativity. James’s general outlook was one of tolerance, and he was a defender of new ideas and nonconformity in various forms.

The brother of the famous novelist Henry James, William was raised in an intellectual household where his father studied ways of combining reason and spirituality. His family traveled between Europe and the United States, and William attended various schools in New York, France, and Switzerland. William began to study art, but switched to science at Harvard. He interrupted his studies with various trips to study in Europe and an expedition to the Amazon.

Always of delicate health, he was too ill to practice medicine when he graduated from Harvard Medical School in 1869. His maladies seem to have been largely psychological, and his health recovered dramatically in April 1870 following his philosophical commitment to the doctrine of free will. In 1872 he was appointed to the faculty of Harvard, where he began his quest to combine philosophy and psychology. His marriage in 1878 to Alice Gibbens seemed to give his life and work new zest. He began to teach religion and ethics in the late 1880s. He began to have health problems again in 1898, and spent two years as a semi-invalid. But by 1907 James was teaching at Columbia University to packed lecture halls; his empirical philosophy was all the rage.