The book, the statue, the sonata, must be gone upon with the unreasoning good faith and the unflagging spirit of children at their play. Is it worth doing? — when it shall have occurred to any artist to ask himself that question, it is implicitly answered in the negative. It does not occur to the child as he plays at being a pirate on the dining-room sofa, nor to the hunter as he pursues his quarry; and the candour of the one and the ardour of the other should be united in the bosom of the artist.

Robert Louis Stevenson (1850–1894) Scottish essayist, novelist, poet

Essay (1888-09), “A Letter to a Young Gentleman Who Proposes to Embrace the Career of Art,” Scribner’s Magazine, Vol. 4, No. 3
    (Source)

Collected in Across the Plains, ch. 10 (1892).