Letters 72.6

The difference, I say, between a man of perfect wisdom and another who is progressing in wisdom is the same as the difference between a healthy man and one who is convalescing from a severe and lingering illness, for whom “health” means only a lighter attack of his disease. If the latter does not take heed, there is an immediate relapse and a return to the same old trouble; but the wise man cannot slip back, or slip into any more illness at all. For health of body is a temporary matter which the physician cannot guarantee, even though he has restored it; nay, he is often roused from his bed to visit the same patient who summoned him before. The mind, however, once healed, is healed for good and all.

Seneca, Letters to Lucilius 72.6, translated by Richard Mott Gummere