What is the source of this Bruce Lee quote?
If you spend too much time thinking about a thing, you’ll never get it done. Make at least one definite move daily toward your goal.
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According to the wikiquote page for this, the origin of the quote is given for the book Striking Thoughts: Bruce Lee's Wisdom for Daily Living, published in 2000 and edited by John Little,
Part III Part II. (Thanks to @ukemi for the correct section reference)
Part III : On Matters of ExistencePart Two - On Being Human - Concepts (Abstracting)
Balance your thoughts with action. — If you spend too much time thinking about a thing, you'll never get it done. p. 43
I am not sure what qualifies to be noticed for google's ngram viewer (Searches book instances for phrases), but this does not come up in that search. Looking at the preview in google books, the placard page gives the information that it is copyright 2000 by Linda Lee Cadwell, so the provenance of it is a little shaky, but it appears that it was Bruce Lee's quote as remembered by LLC and entered in the book.
This quote can be found in Part Two - On Being Human - Concepts (Abstracting) of Striking Thoughts: Bruce Lee's Wisdom for Daily Living (p43):
Balance your thoughts with action. — If you spend too much time thinking about a thing, you'll never get it done.
In the introduction, A Book For Free Spirits, John Little writes this of the sources of the quotes:
The title “Striking Thoughts” is taken from a heading that Bruce Lee created for a series of maxims he wrote down after reading the philosopher Jiddu Krishnamurti’s book First and Last Freedom. However, the committing to paper (and, later, to audiotape) of “striking thoughts” was a habit of Lee’s that extended back to his early days in Hong Kong. Included within the pages of this book are those “striking thoughts” that issued forth in conversations, interviews, and correspondence that Bruce Lee shared with journalism, friend's, and colleagues. Other entries are postulates that Lee had been moved to type out—perhaps with an eye toward one day making use of them—others were quickly jotted down before they escaped his concentration. Still Others were noted in book margins as he read; the result of a “striking thought” that had come to him as he was closely regarding a particular writer or philosopher’s point of view.