Who says a dream is a scripture?

The book's narrator, Adso of Melk, has had a strange and terrible dream about a wild, raucous carnival, where the natural order of the world as understood by medieval Christians has been turned completely upside-down. Adso doesn't know what to make of this vivid, disturbing vision. Fortunately, however, William of Baskerville is on hand to provide an interpretation. He says that Adso's dream is based on an unconscious appropriation of certain elements from the Coena Cypriani, a work forbidden by many of the stricter novice masters, but still widely known in convents and monasteries. William concludes that Adso must have read the story or had it read to him when he was a boy.
The Coena is execrated as a sacrilegious parody of Scripture. Adso doesn't understand why elements of such a notorious work should come to him in his sleep as he's always believed that dreams were divine messages. William explains that, in recent days, Adso has experienced a series of events in which every rule has been upended, so it's hardly surprising that the disturbing, impious images of the Coena should manifest themselves in his dreams.
Adso still can't make much sense of his dream. But William reminds him that dreams should be interpreted allegorically. "Like Scripture?" asks Adso. That's when William replies that a dream is a scripture, and many scriptures are dreams. By this, he means that dreams can reveal important religious truths just as many books of scripture are themselves based on dreams, such as Jacob's ladder in Genesis 28:12.