Mencius - Book 2, Part 1, Chapter 2C

Kung-sun Ch'âu said, 'May I venture to ask an explanation from you, Master, of how you maintain an unperturbed mind, and how the philosopher Kâo does the same?' Mencius answered,'Kâo says, "What is not attained in words is not to be sought for in the mind; what produces dissatisfaction in the mind, is not to be helped by passion-effort." This last, when there is unrest in the mind, not to seek for relief from passion-effort, may be conceded. But not to seek in the mind for what is not attained in words cannot be conceded. The will is the leader of the passion-nature. The passion-nature pervades and animates the body. The will is first and chief, and the passion-nature is subordinate to it. Therefore I say, Maintain firm the will, and do no violence to the passion-nature.'

Ch'âu observed, 'Since you say "The will is chief, and the passion-nature is subordinate," how do you also say, "Maintain firm the will, and do no violence to the passion-nature?"' Mencius replied, 'When it is the will alone which is active, it moves the passion-nature. When it is the passion-nature alone which is active, it moves the will. For instance now, in the case of a man falling or running, that is from the passion-nature, and yet it moves the mind.'

'I venture to ask,' said Ch'âu again, 'wherein you, Master, surpass Kâo.' Mencius told him, 'I understand words. I am skilful in nourishing my vast, flowing passion-nature.'
~ James Legge translation via nothingistic.org ~

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