Meditations 5:26

The mind is the ruler of the soul. It should remain unstirred by agitations of the flesh—gentle and violent ones alike. Not mingling with them, but fencing itself off and keeping those feelings in their place. When they make their way into your thoughts, through the sympathetic link between mind and body, don't try to resist the sensation. The sensation is natural. But don't let the mind start in with judgments, calling it "good" or "bad."