William James divided all believers into two incompatible types

In his Gifford Lectures at Edinburgh in 1901 and 1902, William James carved humanity's religious life in two. The healthy-minded person — Walt Whitman was his example — faces the world with native optimism. Evil, for them, is a kind of mistake, an illusion that dissolves when you look directly at the goodness underlying everything. Their faith is buoyant, almost effortless.

Then there is the sick soul. Tolstoy spent years unable to explain why he should not hang himself. John Bunyan heard a voice tell him he had committed the unpardonable sin. These people cannot simply decide their way out of despair. For them, the bright surface of the world has torn open, and what shows through is dread.

James did not pity the sick souls. He respected them. The healthy-minded view, he argued in The Varieties of Religious Experience, purchases its optimism by ignoring real evidence. Pain, death, and the collapse of everything you trusted are not illusions. The sick soul knows this, feels it in the body, and then — if conversion comes — finds something on the far side of it.

A faith that has passed through despair is, he thought, wider and more honest than one that never needed to.

The lectures became a book in 1902. It remains the most serious attempt by a scientist to treat religious experience as data rather than symptom.