Benjamin Franklin's letter to David Hartley (December 4, 1789) as quoted in "The Writings of Benjamin Franklin," Volume 10 (p. 72) edited by Albert H. Smyth, 1907.
"The Perennial Quest for a Psychology with a Soul: An Inquiry Into the Relevance of Sri Aurobindo's Metaphysical Yoga Psychology in the Context of Ken Wilber's Integral Psychology". Book by Joseph Virtue, 2002.