Harvard psychologist and philosopher William James' The Varieties of Religious Experience: A Study in Human Nature explores the nature of religion and, in James' observation, its divorce from science when studied academically. After publication in 1902 it quickly became a canonical text of philosophy and psychology, remaining in print through the entire century. "Scientific theories are organically conditioned just as much as religious emotions are; and if we only knew the facts intimately enough, we should doubtless see 'the liver' determining the dicta of the sturdy atheist as decisively as it does those of the Methodist under conviction anxious about his soul. When it alters in one way the blood that percolates it, we get the Methodist, when in another way, we get the atheist form of mind."

Author's best-known and most controversial study relates the rise of a capitalist economy to the Puritan belief that hard work and good deeds were outward signs of faith and salvation.