By St. Gregory the Dialogist
(Dialogues, Bk. 4, Chs. 48 & 49)
You must understand that there are six kind of dreams. For sometime they proceed of too much fullness or emptiness of the stomach, sometime by illusion, sometime both by thought and illusion, sometime by revelation, and sometime both by thought and revelation. The two first all by experience know to be true, and the four latter we find mentioned in Holy Scripture.