Dream interpretation was regarded by ancient peoples in Mesopotamia, Egypt, Greece, and Rome as an art requiring intelligence and, sometimes, divine inspiration. It became a motif in literature. It was treated as a science by philosophers and physicians. Dreams were thought to come either as clear messages, or as symbols requiring interpretation. In a method called incubation, the dreamer could sleep in a sacred place in expectation of a dream that would elucidate a problem for which the dreamer desired guidance. Dream-books listing images and their meanings were popular. Historians reported leaders' famous dreams that affected the course of events. Very few ancient writers were skeptical of dreams; Cicero was one. Dream interpretation was an honored profession with exponents such as Artemidorus of Daldis. Ancient dream traditions and beliefs can provide perspective for consideration of more recent theories of dream interpretation.