"In The Presence Of Innocence"

"...what he needed to regain inner peace was not another love affair but a few more times in the presence of innocence. Like a thirsty man finding a spring to drink from, high on a mountainside -"
-Poul Anderson, The Shield Of Time (New York, 1991), pp. 6-7.

But innocence lost is the consistent theme of the Time Patrol series:

Everard and Whitcomb must sabotage Stane's plan for world peace;

Whitcomb succumbs to the temptation to change the past and must leave the Patrol;

Everard, shaken by his encounter with a Danellian, is judged to be "...unfit for steady work..." (Time Patrol, p. 53) and must retrain as an Unattached;

after fourteen years as Cyrus the Great, Keith Denison must relearn how to live in a cramped apartment with Cynthia;

Everard learns that the Patrol itself adjusts the past;

he and Van Sarawak must prevent Deirdre's timeline from coming into existence;

Everard must tell Carl to betray his followers

Everard has learned better than to return to the Midwest of his boyhood;

he remembers a golden summer in Amsterdam...;

Wanda learns the consequences of intervening in past events.

Thus, Everard and his colleagues learn from experience that innocence is transitory.