《韩非子·外储说左上》
狗马是人人皆知的，常常出现在人们眼前，不可能画得很像，所以难；鬼怪是无形的东西，没有在人的面前出现，所以画起来很容易。
夫犬马，人所知也，旦暮罄于前，不可类之，故难。鬼魅无形者，不罄于前，故易之也。
Dogs and horses are known to everyone and often appear in front of people, so it is impossible to paint them very well, so it is difficult. Ghosts and monsters are invisible things, and they do not appear in front of people, so it is easy to draw them.