Rule 4 of 24 · Chapter I — The Laws Beneath the Story
Change one thing, then follow it everywhere
Why this rule exists
The strongest invented worlds usually rest on a single, disciplined lie, one changed variable, whose consequences are then chased honestly through everything they would touch. Suppose the dead can be raised for a price; now follow it. What happens to war, to grief, to inheritance, to the price of a life, to how a mother says goodbye. A world built this way feels deep because it is not a heap of unrelated marvels but a system, where the fantastic element has bent the ordinary world into a new but coherent shape. Readers may never name the logic, but they feel its presence as coherence, the sense that this place was reasoned and not merely decorated.
In practice
Name your central premise plainly, the one big change from our world, and then interrogate it like a stubborn detective. Ask what it does to economics, to religion, to family, to who holds power and who is despised, to childhood and old age and death. Follow the second- and third-order effects, the ones that are not obvious, because those are where the world stops feeling generic. When you are tempted to add a second unrelated marvel, first make sure the first one has been fully spent, since one premise chased all the way down almost always beats three chased halfway.
When it doesn't apply
Rich worlds can hold more than one premise, but each new one multiplies the consequences you must reconcile, so add them knowingly. Some genres, high fantasy, kitchen-sink space opera, delight in abundance rather than austerity. The discipline is not to change only one thing forever, but to follow every change you make all the way to its end.