How Everard Rescues Denison

Poul Anderson, The Guardians Of Time (New York, 1981).

Cyrus the Great reigned for twenty nine years and is an essential part of the timeline guarded by the Time Patrol. Sixteen years into Cyrus' reign, Time Patrolman Manson Everard learns that the Median Harpagus had killed Cyrus as a baby but that when, at a later date, a Cyrus was necessary, Harpagus had forced Time Patrolman Keith Denison into the role. The mysterious arrival of a time traveler was transformed into the providential return of Cyrus.

Denison has had no access to a time vehicle for sixteen years. Now it is impossible for him simply to leave ancient Persia to return to the twentieth century that he remembers having left. A history in which Cyrus did not die in battle after reigning for twenty nine years but disappeared after reigning for sixteen years would not be the history guarded by the Patrol.

Everard's solution: he and Denison travel back further and scare Astyages so much that he does not order Harpagus to kill the infant Cyrus. Thus, the real Cyrus grows up to perform all the deeds that are credited to him in the Bible and in other historical records; Denison is not needed in ancient Persia and can go home.

I think that this means that there are three successive timelines in which:

(i) Cyrus was killed but Denison impersonated him for twenty nine years;
(ii) Cyrus was killed and Denison impersonated him for sixteen years but then disappeared;
(iii) Everard and Denison prevented Cyrus from being killed. 

(i) is the timeline that Everard was in before he traveled back to generate (ii), then (iii).

When Everard asks Denison to guide him to Astyages' court thirty six years previously, Denison suggests:

"'If you're going to change the past anyway, why use me at this point? Come and get me when I'd been Cyrus only one year...'" (p. 119)

Everard refuses for two reasons. The second is:

"'...you're not a suicidal type. Would you actually want the you of this instant never to have existed? Think for a minute precisely what that implies.'" (ibid.)

It implies nothing because it is self-contradictory. It is counter-intuitive though not contradictory to believe that remembered experiences did not happen but it is contradictory to believe that present experiences are not happening - although, of course, they can happen in this timeline though not in a subsequent one. If Everard had followed Denison's suggestion, then the Denison who made the suggestion would have lived for thirteen more years in timeline (i). Everard would have departed to generate two further timelines in which:

(ii) Cyrus was killed and Denison impersonated him for one year but then disappeared;
(iii) as above.

Anderson writes so effectively that he usually conceals the incoherence of the Time Patrol's understanding of causality violation but I think that this incoherence is exposed by Everard's question to Denison, "'Would you want the you of this instant never to have existed?'" (see above)