I just have to explain to... «How to Live Safely in a Science Fictional Universe»

I just have to explain to the customer the basics of Novikovian self-consistency, which no one wants to hear about. No one wants to hear that they went to all this trouble for nothing. For some people, that’s the only reason they rented the thing, to go back and fix their broken lives.
Other people are in the unit all sweaty and nervous and afraid to touch anything because they are so freaked out about the implications of changing history. Oh God, they say, what if I go back and a butterfly flaps its wings differently and this and that and world war and I never existed and so on and yeah.
This is what I say: I’ve got good news and bad news.
The good news is, you don’t have to worry, you can’t change the past.
The bad news is, you don’t have to worry, no matter how hard you try, you can’t change the past.