The Fantastic And The Topical

A mediocre sf author might write a time travel story that is only nominally set in a historical period and that glaringly contradicts itself when presenting time travel paradoxes. By contrast, Poul Anderson, e.g., in "Star of The Sea" and "Amazement Of The World," presents subtle temporal paradoxes in well realized historical periods.

Thus, when Manse Everard of the Time Patrol explores a divergent timeline, the reader is informed of several aspects of life in the twelfth century. See here and here. Further, when Everard compares life in the twelfth and twentieth centuries, this generates a combox discussion of different ways to conduct our affairs in the twenty first century. Thus, we move instantly from the fantastic to the topical.

Larry Niven's Svetz series presents the argument that:

time travel is impossible;
therefore, time travel fiction is fantasy, not hard sf;
therefore, a time traveler seeking an extinct horse/whale etc should find a mythical unicorn/leviathan etc.

This makes sense as the premise of a single series but does not negate the existence of genuine hard sf time travel fiction like Anderson's Time Patrol series! Because the Svetz series is humorous, Niven deliberately includes some inconsistencies in its time travel logic - these might as well exist alongside the unicorns. Meanwhile, because Anderson deals not only with abstruse temporal paradoxes but also with real history, his texts can at any time generate discussion of real, even urgent, issues.