Think and Grow Rich is based on Hill's earlier work The Law of Success, said to be the result of more than twenty years of study of many individuals who had amassed personal fortunes. Hill studied their habits and evolved 16 "laws" to be applied to achieve success. Think and Grow Rich condenses them, providing the reader with 13 principles in the form of a "Philosophy of Achievement".
Mark Hansen has said time has shown that two of the laws/principles are most important:
1) The Master Mind principle/process and
2) "Know very clearly where you want to go."
The book asserts that desire, faith and persistence can propel one to great heights if one can suppress negative thoughts and focus on long-term goals.
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Credit: https://www.youtube.com/watch?v=fX-kcOAdYKs