I was interested in comparing Poul Anderson's Time Patrol with Robert Heinlein's Temporal Bureau. However, I also mentioned James Blish's fictional organization, the Service, which provides another fascinating contrast with the Time Patrol.
Whereas the Service preserves a utopian future paradoxically by creating that future, the Patrol preserves a necessary past, sometimes by causing that past. It is because of the Patrol that Cyrus the Great survived into adulthood and that the Mongols did not invade North America. Despite its futuristic rationale, the Time Patrol series is entirely about the past whereas:
"One of the many original features of this novel is that it does actually concern the future. Most science fiction, if it is not fantasy, is about an extension of the present which only by agreement do we call the future. It catches our attention because we see in it a mirror of the present day. Blish was after something different. Quincunx is like few other fictions, and does not resemble closely anything else Blish wrote."
-Brian W. Aldiss, "PEEP: An Introduction to The Quincunx Of Time" IN James Blish, The Quincunx Of Time (New York, 1983), pp. 6-10 AT p. 6.
Whereas twentieth century Patrolmen visit a succession of historical periods, the founders of the Service, who are already in our future, receive increasingly incomprehensible messages from a succession of further futures with different scientific paradigms. One even hears the commander of a world-line cruiser traveling from 8873 to 8704 eleven million light years away in another galaxy.
Some, not all, writers about time travel address temporal paradoxes with appropriate ingenuity, Poul Anderson being at the top of this list. Blish in "Beep"/Quincunx..., like Gregory Benford in Timescape, shows that such paradoxes can be generated merely by transmitting information backwards in time. Characters in these two novels, by Blish and Benford respectively, face this question: we want to know if it is possible to communicate with the past, we intend to transmit a message that might have been received in the past and we have a record of its reception so what will happen if we now do not send the message?
We have now proceeded from Anderson to Heinlein to Blish to Benford. I have no control over the direction of this blog.