Contrarywise

"To lie," said the Wisest Stone, "is to create worlds. Each falsehood leads to a new universe, a place where it is truth instead, and in turn changes this world by rendering it impossible."

"Lies such as, 'Stones may fly'?" Taku asked.

"Yes. I chose for this world to contain that truth, and thus made it a lie somewhere else."

Taku pondered as they ascended to the next terrace. "How is one to tell a truth from a lie, then?"

"That is the simplest part," the Wisest Stone told him. "When it never occurs, you know it was a lie."