The Varieties of Religious Experience: A Study in Human Nature 
is a book by the Harvard psychologist and philosopher William James that 
comprises his edited Gifford Lectures on "Natural Theology" delivered at the 
University of Edinburgh in Scotland between 1901 and 1902.
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.
These lectures concerned the nature of religion and the neglect of science, 
in James' view, in the academic study of religion. Soon after its publication, 
the book found its way into the canon of psychology and philosophy, and has 
remained in print for over a century. James would go on to develop his 
philosophy of pragmatism, and there are already many overlapping ideas in 
Varieties and his 1907 book, Pragmatism.
-- Excerpted from Wikipedia, the free encyclopedia.