Letters 47.21

I do not wish to delay you longer; for you need no exhortation. This, among other things, is a mark of good character: it forms its own judgments and abides by them; but badness is fickle and frequently changing, not for the better, but for something different. Farewell.

Seneca, Letters to Lucilius 47.21, translated by Richard Mott Gummere