Premises

An sf writer states a premise, then reasons from that premise to generate a story. Poul Anderson's premise of a temporal police force to prevent time travelers from changing the past generates a series of stories about visits to various historical periods where SM Stirling's different premise of a human population mysteriously transported to the remote past generates a trilogy of novels set in what soon becomes an alternative history. Both works assume time travel but, in the case of the Time Patrol series, the time traveling continues throughout the series whereas, in the case of the Nantucket trilogy, it necessarily occurs only once at the beginning of Volume I. Nevertheless, essentially the same conceptual questions arise in both works as recent posts demonstrate.

A writer can also contradict his own premise. It turns out that the real purpose of the Patrol would still have been to hold time to a single course even if there had been no danger of other time travelers changing history. How the Nantucket trilogy pans out remains to be seen at least by this reader.

Stirling's premise of a slave-owning society prospering and becoming powerful precludes the early overthrow of such a society, plausible though such an overthrow would have been. Stirling fearlessly follows his premise all the way to world domination by the Draka although he does allow other human beings to remain free off Earth and in alternative universes - which gives us a link to the premise of the Nantucket trilogy.