The Self-Consistency Principle
I'm writing a novel that deals with time travel. I've long needed a rule set that guides the laws and constraints I must live within, and the Novikov self-consistency principle — developed by Russian physicist Igor Dmitriyevich Novikov — is proving to be extremely helpful.
The concept is simple: a time traveler has agency to do anything they want. However, the probability of them successfully executing an action that creates a paradox is zero.
For example, if Bob goes back in time and attempts to prevent his grandparents from meeting, nothing will stop him from doing so, he will just fail. Thus, from my book's perspective, once an event occurs, it is set and cannot be undone, even by using time travel. This gives me a tight framework which I have been using to constructively limit what my characters can and cannot achieve. Without it, I was going a little crazy, because time travel makes almost anything possible in a way that collapses every action into meaninglessness.