Everything seems fine. Everything seems better than fine. Your life is going great. You’re happy. You’re in love. Your finances are great. But will it last? Or will Fortune, as Seneca said she is wont to do, surprise you with a reversal?

“Could you have seen, had you been walking on Amsterdam Avenue and caught sight of the bridal party that day, how utterly unprepared the mother of the bride was to accept what would happen before the year 2003 had even ended?,” Joan Didion writes hauntingly in her beautiful book Blue Nights (a must read). “The father of the bride died at his own dinner table? The bride herself in an induced coma, breathing only on a respirator, not expected by the doctors in the intensive care unit to live the night? The first in a cascade of medical crises that would end twenty months later with her own death?”

No one can see what Fortune has in store for us, the Stoics remind us. And that’s the point. We are in the dark. We are not in control. We live in an unwalled city. We are vulnerable. Seneca knew this first hand–exiled once by unexpected illness, another time by the whim of the emperor, and a third time, a knock on the door signaled that his death sentence had been handed down.

We cannot take the present moment for granted. Because the future that lies before us is uncertain. Life itself is uncertain. Live accordingly–or rather, as Seneca said, live immediately.