Elias spent his life following the equations in the notebook. He built the Chronos Engine, a device capable of piercing the veil of the multiverse. But as the engine roared to life, the Library began to burn. The friction of the time-fissure ignited the ancient scrolls.

For years, Elias had been haunted by a memory: a silver-haired man giving him this very notebook when he was just a boy. "The world ends in fire," the man had whispered. "Unless you build the Machine."

“The causal nexus divided when Nyssa changed history... causing the universe to exist in two quantum states and releasing blinovitch energy.” Tardis | Fandom

“Wally learns the hard way that good intentions don't off set the consequence of time travel... Wally's actions have already set off a chain of events that can't be stopped.” Fanfiction.net

He didn't have a choice. To ensure he grew up to build the machine that would bring him here, he had to give the boy the book. He had to be the spark that started the fire.

The concept is widely explored in fiction and philosophy as a way to understand our interconnected reality.

"The world ends in fire," he whispered to his younger self. "Unless you build the Machine." Perspectives on Causal Nexus