In this chapter, I first provide an historical introduction on what a thought experiment is by citing various popular examples from science and philosophy. I then propose that we should identify a simple thought experiment with a story/question pair that aims at testing a certain hypothesis whose question must be generalizable to a scientific or a philosophical question. Next I discuss the controversy concerning the divergence between our "intuitions" in answering such questions depending on our culture and how this has recently given rise to "experimental philosophy". I then take up complex thought experiments. I argue that a thought experiment may have "negative heuristics" as it may help us reveal how our concepts may break down when applied to areas outside their ordinary domain.