Mencius - Book 6, Part 1, Chapter 2

The philosopher Kâo said, 'Man's nature is like water whirling round in a corner. Open a passage for it to the east, and it will flow to the east; open a passage for it to the west, and it will flow to the west. Man's nature is indifferent to good and evil, just as the water is indifferent to the east and west.'

Mencius replied, 'Water indeed will flow indifferently to the east or west, but will it flow indifferently up or down? The tendency of man's nature to good is like the tendency of water to flow downwards. There are none but have this tendency to good, just as all water flows downwards.

'Now by striking water and causing it to leap up, you may make it go over your forehead, and, by damming and leading it you may force it up a hill; but are such movements according to the nature of water? It is the force applied which causes them. When men are made to do what is not good, their nature is dealt with in this way.'

~ James Legge translation via nothingistic.org ~

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