“That painter who does not doubt learns little. When the work surpasses the judgement of the worker, that worker acquires little and when the judgement surpasses the work, that work never cease to grow better, unless avarice prevents it.“~ Leonardo Da Vinci
I could not find the original author who found this quote. The description states that it is from Da Vinci’s own Trattato della Pittura standard edition of H. Ludwig, Vienna 1882, paragraph number 62.
There is a similar quote by Michelangelo but I prefer Da Vinci’s.
“A man paints with his brains and not with his hands.“ ~ Michelangelo