James did important work in philosophy of religion. In his Gifford Lectures at the University of Edinburgh he provided a wide-ranging account of The Varieties of Religious Experience (1902) and interpreted them according to his pragmatic leanings. Some of the important claims he makes in this regard:
- Religious genius (experience) should be the primary topic in the study of religion, rather than religious institutions—since institutions are merely the social descendant of genius.
- The intense, even pathological varieties of experience (religious or otherwise) should be sought by psychologists, because they represent the closest thing to a microscope of the mind—that is, they show us in drastically enlarged form the normal processes of things.
- In order to usefully interpret the realm of common, shared experience and history, we must each make certain "over-beliefs" in things which, while they cannot be proven on the basis of experience, help us to live fuller and better lives.
James investigated mystical experiences throughout his life, leading him to experiment with chloral hydrate (1870), amyl nitrite (1875), nitrous oxide (1882), and peyote (1896). James claimed that it was only when he was under the influence of nitrous oxide that he was able to understand Hegel. He concluded that while the revelations of the mystic hold true, they hold true only for the mystic; for others, they are certainly ideas to be considered, but can hold no claim to truth without personal experience of such. American Philosophy: An Encyclopedia classes him as one of several figures who "took a more pantheist or pandeist approach by rejecting views of God as separate from the world."