Final Gentleman

Story, first published: The Magazine of Fantasy and Science Fiction, Jan. 1960

A famous writer, the last surviving "gentleman," is phasing out his career. In a magazine interview occasioned by his retirement, he learns that certain facts about his life no longer seem to be true. When he goes to his mother's for dinner, he finds a stranger living there. Other evidence points to the fact that he is not really who he seems to be, that his gentleman's existence is imagined and not real. He has the ability to literally put himself in anyone else's situation by the power of his imagination. He remembers a deal he had made thirty years ago with a magazine book editor to play this double role to have influence on others who could drastically alter the course of world events.
The writer goes to the magazine as his shabbier real self for a showdown. At the magazine is "Harvey", a Jeanne-Dixon-like computer whose predictions have been amazingly right. The writer discovers that the editor is an alien who has been controlling humans (another "Harvey" was the Delphic oracle) but only at "crisis points" in human history. The writer becomes a Neanderthal and tears the alien apart; when he does, "Harvey" disappears. He concludes that humanity has a right and duty to be itself, without outside interference.

Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.62-63

A well-known writer approaches retirement, and becomes aware that certain aspects of his life have been a delusion forced upon him in order that he may be the influential writer that the Powers That Be feel he must be, so as to set the proper societal ripples in motion. The Powers That Be turn out to be a massively powerful AI, and the creature that maintains it while disguised as an ordinary human.

Brendan: Balancing Frogs