You are astonished to find yourself the butt of so much calumny, opposition, indifference and ill-will. You will be more so and have more of it; it is the reward of the good and the beautiful: one may calculate the value of a man from the number of his critics and the importance of a work by the evil said of it.

Gustave Flaubert (1821-1880) French writer, novelist

Letter to Louise Colet (14 Jun 1853) [tr. Hannigan (1896)]
    (Source)

Alternate translation: "You can calculate the worth of a man by the number of his enemies, and the importance of a work of art by the harm that is spoken of it." [Source]