"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)