Mencius - Book 5, Part 2, Chapter 1A

Mencius said, 'Po-î would not allow his eyes to look on a bad sight, nor his ears to listen to a bad sound. He would not serve a prince whom he did not approve, nor command a people whom he did not esteem. In a time of good government he took office, and on the occurrence of confusion he retired. He could not bear to dwell either in a court from which a lawless government emanated, or among lawless people. He considered his being in the same place with a villager, as if he were to sit amid mud and coals with his court robes and court cap. In the time of Châu he dwelt on the shores of the North sea, waiting the purification of the kingdom. Therefore when men now hear the character of Po-î, the corrupt become pure, and the weak acquire determination.

'Î Yin said, "Whom may I not serve? My serving him makes him my sovereign. What people may I not command? My commanding them makes them my people." In a time of good government he took office, and when confusion prevailed, he also took office. He said, "Heaven's plan in the production of mankind is this: that they who are first informed should instruct those who are later in being informed, and they who first apprehend principles should instruct those who are slower in doing so. I am the one of Heaven's people who has first apprehended; I will take these principles and instruct the people in them."

'He thought that among all the people of the kingdom, even the common men and women, if there were any who did not share in the enjoyment of such benefits as Yâo and Shun conferred, it was as if he himself pushed them into a ditch; for he took upon himself the heavy charge of the kingdom.
~ James Legge translation via nothingistic.org ~

Go here to read the introductory post to this serialized version of the Works of Mencius.