They

     It is true that Philip K. Dick was well known, perhaps even had the market cornered on the world-isn't-what-it-appears-to-be story in books like Time Out of Joint (to name just one among many). However, another science fiction legend, Robert Heinlein, penned a story that employs this same existential paranoia to great effect. Though mainly known for his trenchant social commentary (as in Stranger in a Strange Land and Starship Troopers), his short story They borrows a page from the philosopher Descartes (he of the seminal "I think therefore I am") and manages to pick apart the details of everyday life - details so common we all tend to dismiss them without thought - and tie them into a sinister conspiracy on such a grand scale, Mr. Dick would have been proud. Along with Mr. Heinlein's own the Unpleasant Profession of Jonathan Hoag, They was a watershed moment in sci-fi and horror literature of the day, changing the course of the genre forever. All in just ten pages.