“The greatest discovery of all time is that a person can change his future by merely changing his attitude.”

Session 47
The Early Sessions: Sessions 1-42, 1997, The Early Sessions: Book 2

As quoted in Life In the Open Sea (1972) by William M. Stephens, p. 21.
1970s and later

"Man alone, of all creatures of earth, can change his thought pattern and become the architect of his destiny." Actually said by Spencer W. Kimball, twelfth president of The Church of Jesus Christ of Latter-day Saints, in his Miracle of Forgiveness (1969), p. 114. This predates any of the misquotations.
Other forms: "The greatest discovery of my generation is that human beings can alter their lives by altering their attitudes of mind." This is also misattributed to Albert Schweitzer.
James did say: "As life goes on, there is a constant change of our interests, and a consequent change of place in our systems of ideas, from more central to more peripheral, and from more peripheral to more central parts of consciousness."
Misattributed
Context: Man alone, of all the creatures on earth, can change his own patterns. Man alone is the architect of his destiny. The greatest revolution in our generation is the discovery that human beings, by changing the inner attitudes of their minds, can change the outer aspects of their lives … It is too bad that most people will not accept this tremendous discovery and begin living it.

Source: The Best of All Possible Worlds (2006), Chapter 8, The End of Nature, p. 150.

Pg. 306-307
Against Method (1975)
Context: Combining this observation with the insight that science has no special method, we arrive at the result that the separation of science and non-science is not only artificial but also detrimental to the advancement of knowledge. If we want to understand nature, if we want to master our physical surroundings, then we must use all ideas, all methods, and not just a small selection of them. The assertion, however, that there is no knowledge outside science - extra scientiam nulla salus - is nothing but another and most convenient fairy-tale. Primitive tribes has more detailed classifications of animals and plant than contemporary scientific zoology and botany, they know remedies whose effectiveness astounds physicians (while the pharmaceutical industry already smells here a new source of income), they have means of influencing their fellow men which science for a long time regarded as non-existent (voodoo), they solve difficult problems in ways which are still not quite understood (building of the pyramids; Polynesian travels), there existed a highly developed and internationally known astronomy in the old Stone Age, this astronomy was factually adequate as well as emotionally satisfying, it solved both physical and social problems (one cannot say the same about modern astronomy) and it was tested in very simple and ingenious ways (stone observatories in England and in the South Pacific; astronomical schools in Polynesia - for a more details treatment an references concerning all these assertions cf. my Einfuhrung in die Naturphilosophie). There was the domestication of animals, the invention of rotating agriculture, new types of plants were bred and kept pure by careful avoidance of cross fertilization, we have chemical inventions, we have a most amazing art that can compare with the best achievement of the present. True, there were no collective excursions to the moon, but single individuals, disregarding great dangers to their soul and their sanity, rose from sphere to sphere to sphere until they finally faced God himself in all His splendor while others changed into animals and back into humans again. At all times man approached his surroundings with wide open senses and a fertile intelligence, at all times he made incredible discoveries, at all times we can learn from his ideas.

Source: A Joseph Campbell Companion: Reflections on the Art of Living