There is a strange mercy in imagining, on an ordinary day, that you have already lost what you love most. Not to wallow, and not to brace for the worst, but because the mind has a quiet habit that robs every life: it stops seeing whatever it has grown used to. Picturing the loss, briefly and on purpose, makes the thing visible again.
The practice
Praemeditatio malorum, the premeditation of adversity, is the deliberate rehearsal of loss, difficulty, or failure before any of it arrives. You imagine the plan falling through, the journey going wrong, the people you love no longer here. You hold it clearly for a moment, and then you return to a present that is suddenly, vividly intact. Seneca practised it not to spoil his days but to stop sleepwalking through them.
"We suffer more often in imagination than in reality." — Seneca
Running the treadmill backwards
This is the Stoic answer to a trap the mind sets for everyone: we adapt to whatever we have. The first morning in a new home you notice every window; a year later you notice none of them. Psychology has a name for the mechanism, the hedonic treadmill, and a good deal of evidence that it flattens almost every gain back to a baseline of wanting more. Negative visualisation runs that treadmill backwards. By picturing a thing gone, you feel its presence again, and gratitude returns with it, gratitude being little more than clear sight of what is already enough. It is the deliberate cousin of Santosha, the contentment that comes from noticing you already have what you need.
The remembrance of death
Its sharpest and most famous form aims the practice at the one loss that is certain. Memento mori, "remember that you must die", is negative visualisation turned toward your own life, and met as a spur to living rather than a source of dread it becomes one of the most clarifying practices there is. Because it is really about how to live now, its fuller treatment lives on Death and impermanence, where the Stoic memento mori stands beside the Buddhist and yogic ways of holding the same truth.
Not pessimism
It is worth saying plainly what this is not. Negative visualisation is not pessimism, which expects the worst and moves in. It is closer to the opposite: a brief, chosen visit to the worst, in the service of loving the present more honestly. You go to the imagined loss so you do not have to wait for the real one to teach you what you had. Held lightly, it is one of the gentlest ways the Stoics found to step off the treadmill of wanting and into the life that is already here.