Scientists, philosophers, visionaries, and creative writers around the world—in all eras, traditions, and cultures—have often turned to a fascinating form of exploratory inquiry that we today call thought experiments. These short, creative, often wildly outlandish scenarios allow people to ponder some of our most urgent and life-changing questions: How is the universe structured? What is time? By what methods do we arrive at our most deeply held beliefs about self and consciousness? Are there absolute standards of right conduct? Can we be sure that the world around us is actually real—and does that even matter?