Some good must come by clinging to the right. Conscience is a man's compass, and though the needle sometimes deviates, though one often perceives irregularities in directing one's course by it, still one must try to follow its direction.
Source, in full
Quote in a letter of Vincent to brother Theo, from The Hague, between c. 13 and c. 18 December 1882; as cited in Dear Theo: the Autobiography of Vincent Van Gogh; ed. Irving Stone and Jean Stone, (1995) p. 181 - ISBN 0452275040