Frederick Pohl: The Tunnel Under the World: Pinching yourself is no way to see if you are dreaming. Surgical instruments? Well, yes-but a mechanic's kit is best of all!

This famous Pohl story explores cybernetic robots and implanted personalities (long before they were cool) in a way that certainly expanded my way of looking at reality. Is that wall really a wall? or is it just kinda, sorta real? And who am I? The protagonist, Guy Burckhardt, wakes up screaming from a horrible dream of explosions, searing fire, choking gas and other terrible ways to die. But he wakes up so it must have been just a bad nightmare, right? To find out that piece of information...