Rule 2 of 24 · Chapter I — The Laws Beneath the Story
Decide what cannot happen
Why this rule exists
A world where anything is possible is a world where nothing matters, because stakes are made entirely of limits. If the wizard can always conjure a door, no wall is ever a threat; if the ship can always outrun trouble, no chase is ever tense. Impossibility is not a failure of imagination but the frame that lets imagination mean something. The most vivid invented worlds are defined as much by their walls as by their wonders, and readers feel those walls even when they are never named, in the weight of a locked door or the dread of a river that cannot be crossed before dark.
In practice
Before you list what your world can do, list what it cannot. Name the hard boundaries: what magic will never fix, what technology never reached, what the dead do not do, what no amount of will can buy back. Make those limits load-bearing, so that characters must scheme and suffer around them rather than through them. When a scene feels weightless, look for the missing wall and put it back. Resist the late-story temptation to quietly demolish a limit because it has become inconvenient; that convenience is precisely the cost you agreed to pay for the tension it once bought you.
When it doesn't apply
Some stories thrive on near-omnipotence and put their stakes elsewhere, in moral cost, in relationship, in the loneliness of power. And a limit can be broken as a genuine climax if the whole story has been about earning that break. What you cannot afford is a limit that dissolves quietly, off the page, the instant it stops being useful.