As a man thinketh, so is he.

You've heard it from motivational speakers. You've heard it from self-help gurus. You've heard it from mindset coaches! There's even a whole book with this as a title. It's a quote (actually a misquote) from the Bible--Proverbs 23:7. 

Those who quote this from the Bible often leave out 3 important words that take it from being a true statement to one that's not completely false, but not completely true either.