Toward the end of "The Pit and the Pendulum," the narrator tells us that he could no longer believe that what he saw was "unreal" (35). But, as readers, it's hard to make that same leap – though the words may be fixed on the page, the story itself seems fundamentally unstable. It takes place in an "other space," one in which even the simplest things – the shape and size of a room – cannot be established. And, well, if even that sort of thing is questionable, how can we trust anything at all?
In "The Pit and the Pendulum," reality is a fluid thing: our narrator can be sure of nothing so long as the inquisitors control him.
What's happening to our narrator is definitely real: Poe's use of sensory descriptions drives that point home.