"Whoever wants to portray something that does not exist has to obey certain rules. Those rules are more or less the same as for the teller of fairy tales: he has to apply the function of contrasts; he has to cause a shock.… That is why such a game can be played and understood only by those who are prepared to penetrate the surface, those who agree to use their brains, just as in the solving of a riddle. It is thus not a matter for the senses, but rather a cerebral matter. Profundity is not at all necessary, but a kind of humour and self-mockery is a must."
(Escher, in Escher, 1989, p. 136)