Several days later, Tomas posits that somewhere out in space, there is a planet where everyone gets to be born again. They will retain all the information about their first life, and get to try again. Then there is another planet, where they get to try the third time. And so on. This is Tomas's vision of eternal return (5.16.4).
Now, says the narrator, we have real definitions of pessimism and optimism.
An optimist is a person who thinks that on planet number five, the history of mankind will be less bloody. A pessimist is someone who thinks otherwise.