Epictetus the stoic - Discourses, Book 2, Chapter 15
To or against those who obstinately persist in what they have determined
When some persons have heard these words, that a man ought to be constant, and that the will is naturally free and not subject to compulsion, but that all other things are subject to hindrance, to slavery, and are in the power of others, they suppose that they ought without deviation to abide by everything which they have determined. But in the first place that which has been determined ought to be sound. I require tone in the body, but such as exists in a healthy body, in an athletic body; but if it is plain to me that you have the tone of a frenzied man and you boast of it, I shall say to you, "Man, seek the physician": this is not tone, but atony.