Mystery
Edgar Allan Poe takes the cake when it comes to mystery. From houses spontaneously combusting ("The Fall of the House of Usher") and letters going missing ("The Purloined Letter") to bizarre hallucinations ("The Tell-Tale Heart"), mysteries of all kind pervade Poe's works.
"A Dream Within a Dream" is Poe's exploration of the ultimate mystery: reality. In the poem, the question of whether or not the things we see and seem are actually there and real remains unanswered. By the end of the poem, the speaker is no closer to solving the question of the nature of reality. While at first he seems sure of it, he concludes by essentially throwing up his hands, unable to solve the riddle. Hmm, that's kind of how we usually feel at the end of a Poe story. Maybe he knew what he was up to all along.