Philosophy
The Metaphysics of Personal Identity Across Possible Worlds
Quick fact
Philosopher David Lewis famously argued that you do not exist in any other possible world—only in the actual world—and that when we imagine an alternative life, we are really imagining a 'counterpart' who resembles you, not you yourself.
Why this is interesting
We're used to wondering what might have been—if you'd taken that job, if you'd never met your partner. But what does it even mean to say that it's you in that other scenario?