Letters 44.6

Suppose, then, that you were not a Roman knight, but a freedman, you might nevertheless by your own efforts come to be the only free man amid a throng of gentlemen. “How?” you ask. Simply by distinguishing between good and bad things without patterning your opinion from the populace. You should look, not to the source from which these things come, but to the goal towards which they tend. If there is anything that can make life happy, it is good on its own merits; for it cannot degenerate into evil.

Seneca, Letters to Lucilius 44.6, translated by Richard Mott Gummere