Thought experiments have long been a vital part of the creative, intellectual process in modern science-and, by extension, so have "demons." Demons are hypothetical beings imagined by scientists to perform specific roles within thought experiments-embodying special powers or abilities and personifying tough intellectual challenges or highlighting apparent paradoxes. They are used as a way of exploring what would happen if one fiddled with or upset the sturdiest of physical laws, or experimented with physical or natural processes or phenomena in ways that the scientist imagining them otherwise could not. As such, they help clarify the limits of what is possible in the physical world, or show weaknesses in our understanding of an observable phenomenon, or highlight cracks in a hypothesis or theory. Unencumbered by the physicality of our concrete world, demons are thus useful to scientists in their intellectual quest to understand how nature works, and in the creative exploration of the frontiers of science"--