Letters 50.8

We should therefore proceed to the task of freeing ourselves from faults with all the more courage because, when once committed to us, the good is an everlasting possession; virtue is not unlearned. For opposites find difficulty in clinging where they do not belong, therefore they can be driven out and hustled away; but qualities that come to a place which is rightfully theirs abide faithfully. Virtue is according to nature; vice is opposed to it and hostile.

Seneca, Letters to Lucilius 50.8, translated by Richard Mott Gummere