Template:Anthropic Principle
For any given story, there exist basic elements that are required for the story itself to happen; there would be no story otherwise.
The original Anthropic Principle is a theoretical explanation of why the conditions of the universe are so perfect for the existence of intelligent life (like us humans on Earth). Why? Because without those conditions, we wouldn't even be here to be making those observations in the first place. Even though the raw probability of those conditions is astronomically unlikely, our very existence requires us to accept that it must have happened somewhere.
The Anthropic Principle as it applies to fiction is similar: Every fictional universe has fundamental, axiomatic elements without which its story simply could not exist, and the reader must accept those elements in order to enjoy the work. The ultimate expression of this trope is Minovsky Physics -these elements are actually carefully planned in advance, ensuring a logical transition from real life to the fictional universe.