Invited to give the Gifford Lectures at Edinburgh, James made a methodological choice that created a discipline. He would not discuss the truth of religious doctrines, and he would not begin with churches, since institutions are second hand by the time they exist. He would begin with what he called the feelings, acts and experiences of individual men in their solitude, and he would take his evidence from what those individuals wrote down.
So the book proceeds through documents: conversion accounts, letters, journals, the autobiographies of the devout and of the wretched. Out of them he draws distinctions that are still in use. There is the religion of healthy mindedness, which finds the world good and evil unreal, and there is the sick soul, for whom the world's horror is the primary datum and any religion that ignores it is trivial. There is the once born and the twice born temperament. And there are four marks by which he proposes to identify mystical states of consciousness: they are ineffable, resisting adequate report; they are noetic, presenting themselves as knowledge rather than emotion; they are transient; and they are passive, arriving rather than being achieved.
His conclusion is careful and characteristically pragmatic. The value of a religious experience cannot be settled by its cause, since a state of mind is not refuted by an account of the nerves that produced it. It is to be judged by its fruits: what it does to the person who has it, over time, in life.