Skepticism about dream interpretation and the divinatory power of dreams is hardly a modern innovation, although few ancients were as outspokenly contemptuous as Diogenes the Cynic in the fourth century bce. He is said to have remarked that when he saw dream interpreters, prophets and those who attend to them, he thought there was nothing more foolish than man (Diogenes Laertius 6.24), and is said to have told those who got worked up about their dreams that they did not pay attention to what they did while awake but were very curious about what they imagined while asleep (6.43). Since many contemporary academics share Diogenes's skepticism (and perhaps his contempt), it may be difficult today to grasp just how seriously dreams could be taken in antiquity. Countless devotees of Asclepius followed the god's dream prescriptions to cure their maladies; a dream led Galen's father to direct his son to study medicine; and several authors, including Pliny the elder, Cassius Dio, Artemidorus and Synesius, were inspired to write by dreams. And one dream may have played an important part in a pivotal moment of ancient history.

It would be overdramatic to claim that Constantine's dream before the battle of the Milvian Bridge, October 28, 312, was responsible for his conversion to Christianity and thus for the eventual establishment of Christianity as the dominant religion of the Roman empire. Nevertheless, if Constantine really did have this dream, it surely did contribute to his belief in the power of the Christian god and might be the most consequential dream in Western history. Even if one regards the dream as a fiction, however, it attests to the pervasive ancient belief in godsent dreams that foretell momentous events. Constantine's is an example of a type that goes back to Homer: dreams that promise victory to generals. Constantine may not have had this dream but decided to encourage his troops before the battle by declaring that he had (W. V. Harris 2005a).

Eusebius, bishop of Caesarea, who wrote his Life of Constantine toward the end of his own life (d. 339), asserted that Constantine had told him about a vision and dream long after their occurrence and had confirmed his account with an oath (1.28).