The Master Key System is a personal development book by Charles F. Haanel that was originally published as a 24-week correspondence course in 1912. The course is a 'practical wrapper' for the seven hermetic principles as laid out in the Kybalion. Each chapter contains a sequentially numbered section to read, which includes an exercise towards the end, followed by a section with questions and answers. It has had a huge influence on successive New Thought writers, particularly those preaching 'Prosperity Consciousness.' The 'Law of Attraction' at the heart of Haanel's system has been acknowledged as one of the primary sources for The Secret.