Every evening apparently before our eyes the sun goes down behind the stationary horizon, although we are well aware that the sun is fixed and the horizon moves. We are unable to eliminate this optical illusions by reasonably convincing ourselves that our eyes are deceiving us: obstinately and unswervingly, the mechanism follows its own rule and thus wields an imperious mastery over the mind. The formation of visual impressions, Helmholtz realized, is achieved primarily by unconscious judgments. Through our eyes, we necessarily perceive things as real, for the results of the unconscious conclusions are interpretations which "are urged on our consciousness, as if an external power had constrained us, over which our will has no control".