01
The idea
The practice is straightforward and counterintuitive: set aside time to imagine, specifically and in detail, the loss you are most avoiding. Not to make peace with it and not to accept it — to convert it from a shadow into an event with a shape.
Seneca goes further than imagination. He recommends periodically living as if ruined for a few days — the plainest food, the roughest bed — and then asking the question the whole exercise is built for: is this what I was afraid of?
What the practice removes is not the badness of the event. It removes the ambush. Most of the destructive force of a bad event comes from arriving in a mind that had ruled it out, and the rehearsal is an inoculation against exactly that.
02
Why this matters
The events that damage people most are rarely the worst ones. They are the ones nobody had ever considered — the loss that was structurally impossible in the person's model of their life until the morning it happened.
Rehearsal also reduces the gap between how bad a thing is and how bad it feels. A rehearsed loss is bad. An unrehearsed loss of the same size is bad plus disorienting plus humiliating plus proof that you were a fool not to see it.
And it produces gratitude as a side effect rather than as an instruction. Having genuinely sat with the absence of something, its presence is briefly and accurately visible. This is far more reliable than being told to appreciate things.