Along with many people before and since, most Greeks and Romans believed that dreams could give information about past, present, and even future events. Dreams, however, are manifestly not records or transcriptions of these events.
Dreams demand interpretation.
The need to distinguish true dreams from false leads naturally to consideration of the causes and mechanisms of dreaming. Dreams could be caused by external factors like a god, a ghost, or a daemon, or by internal factors like the dreamer's own soul, recollection of waking activities, or physiological state. Around these fundamental polarities of true vs. false and divine vs. natural, ancient philosophers, writers, and theorists of all kinds organized their thought about dreams. For most people, the most important function of dreams was to predict the future, and the first step in interpreting dreams was to recognize which dreams were both god-sent and true. Whatever dreams are divine and foretell evil or good to communities or to private persons have interpreters who are in possession of an art concerning such matters.