The Creature from Cleveland Depths by Fritz Leiber, Science Fiction, Fantasy, Short Stories

Leiber sets the tale in a future when "missiles are on the prowl," and most people live underground. George Gusterson is a writer with crazy ideas -- one being, he still lives on the surface. For another, he imagines a gizmo that would remind him of things like when to turn on the TV. George's mere whim inspires an actual gadget called the Tickler, just a "wire recorder and clock" at first, but then . . . it whispers constantly through an earphone. It instills positive thinking. It injects drugs. It makes decisions. It weights 28 pounds. And it won't get off. Only Gusterson understands what "the little fellow perched on your shoulder" is really saying, one word: Obey! And only Gusterson knows what to say back, if it's not too late.