If something in you yourself says “you aren’t a painter” — IT’S THEN THAT YOU SHOULD PAINT, old chap, and that voice will be silenced too, but precisely because of that.

Vincent van Gogh (1853–1890) Dutch painter

Letter to Theo van Gogh (28 Oct 1883)
    (Source)

Alt. trans.: "If you hear a voice within you say 'you cannot paint,' then by all means paint, and that voice will be silenced."