What kind of thinking do we do when we sleep? Is that what dreaming is? And how is it different from the thinking we do in waking life? In this episode of “What Is X?,” Justin E.H. Smith talks to Matthew Spellberg, a scholar working on a comparative history of dreaming, about one of the mysteries that has fascinated humankind from the ancients to Descartes and Freud and beyond: What are dreams, really? Why do we have them? Why do we share them? And what can they tell us?