Letters 74.25

Virtue suffers no space in us to be unoccupied; it takes possession of the whole soul and removes all sense of loss. It alone is sufficient; for the strength and beginnings of all goods exist in virtue herself. What does it matter if running water is cut off and flows away, as long as the fountain from which it has flowed is unharmed? You will not maintain that a man’s life is more just if his children are unharmed than if they have passed away, nor yet better appointed, nor more intelligent, nor more honourable; therefore, no better, either. The addition of friends does not make one wiser, nor does their taking away make one more foolish; therefore, not happier or more wretched, either. As long as your virtue is unharmed, you will not feel the loss of anything that has been withdrawn from you.

Seneca, Letters to Lucilius 74.25, translated by Richard Mott Gummere