《墨子·47章 贵义》
墨子说： 淄水之南的人不能渡淄水北去，淄水之北的人也不能渡淄水南行，他们的脸色有黑的有白的，为什么都不能渡呢？
子墨子曰： 南之人不得北，北之人不得南，其色有黑者，有白者，何故皆不遂也？
Mozi said: People south of the Zi River cannot cross it to go north, and people north of the Zi River cannot cross it to go south. Their complexions are some dark and some fair—why is it that neither of them can cross?