“What the mind can conceive and believe, and the heart desire, you can achieve.”

Actually said by Napoleon Hill, Stone later added the line "...with P.M.A." (Positive Mental Attitude) to the end of this quote.
Misattributed
Variant: Whatever the mind of man can conceive and believe, it can achieve.

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Variant: Whatever the mind of man can conceive and believe, it can achieve.
Source: Think and Grow Rich: A Black Choice

Part 6 "Beyond System — The Ultimate Source of Jeet Kune Do"
Jeet Kune Do (1997)

As quoted in Self-Motivation Through Risk Taking! : Are You Leading Or Do You Wither with Problems? (2005) by M. Nadarajan Munisamy

Will
Source: I am That, P.119.

Speech at Anderson College in Anderson, Indiana (4 March 1979), quoted in Psychology Through the Eyes of Faith (1987) by David G. Myers and Malcolm A. Jeeves. The first sentence is a modification of a quote by Napoleon Hill: "Whatever the mind of man can conceive and believe, it can achieve."