The artist's feeling is his law. Genuine feeling can never be contrary to nature; it is always in harmony with her. But another person's feelings should never be imposed on us as law. Spiritual affinity leads to similarity in work, but such affinity is something entirely different from mimicry. Whatever people may say of Y's paintings and how they often resemble Z's, yet they proceed from Y and are his sole property.

from his writings Thoughts on Art, Caspar David Friedrich; as quoted in Letters of the great artists – from Blake to Pollock, Richard Friedenthal, Thames and Hudson, London, 1963, p. 32