Otto thinks he's the only one in the world, and has difficulty believing the existence of anything outside of his own brain. Meanwhile, Ian's brain is trying to fully integrate that he's living one of 7+ billion similarly rich existences. 

Otto: Ian, I'm not sure you exist.

You'll want to listen to this one first. 

But. If time has no basis in physics... What does it mean to be first?

This explains who we are and what we're doing.... in a less existential, more concrete way, for once.