Lao-tzu stopped at the border by customs
In 'Legend of the Origin of the Book Tao-te-ching', the aged Lao-tzu is leaving his country on an ox when a poor customs man stops him and asks what he taught; told that 'soft water wears down the mighty stone', the official insists the wisdom be written down before he goes. The poem credits the book's existence to the customs man who extracted it: 'wisdom has to be wrested from the wise.'