This short tract on the power of thought was the second book written by New Thought author James Allen and was first published in 1903. It has never been out of print and is now considered one of most influential self help texts of all time.

The title is taken from a verse in Chapter 23 of the Book of Proverbs: "As a man thinketh in his heart, so is he."

Allen's central thesis is that thought is not just a vehicle for comprehending and explaining the world, but is also the prime mechanism for creating and modifing human experience.