Saint John Climacus. "The Ladder of Divine Ascent". Step 3.
Concerning the dreams of novices
Our mind is the instrument of knowledge, but it is very imperfect and filled with all sorts of ignorance. This is a fact that cannot be disguised.
Now the palate discriminates between various kinds of food, the hearing distinguishes between the things it perceives, the sun shows up the weakness of the eyes, and words reveal the ignorance of a soul. Nevertheless, the law of love urges us to reach beyond ourselves, and so it seems to me—and I do not wish to be insistent—that, immediately after this discussion of exile, or rather, in the course of it, something ought to be said about dreams. For we should not be unaware of this type of deceit practiced by our wily enemies.
A dream is a stirring of the mind during the body's rest, while a fantasy is something that tricks the eyes when the intellect is asleep. Fantasy occurs when the mind wanders, while the body is awake. A fantasy is the contemplation of something that does not actually exist.
It must be clear why I have decided to speak here about dreams. After we leave home and family for the sake of the Lord, after we have gone into exile for the love of God, the demons try to shake us with dreams. They show us our relatives grieving, near death, poverty-stricken or imprisoned because of us. But the man who believes in dreams is like someone running to catch up with his own shadow.
The devils of vainglory do their prophecies in dreams. They guess the future and, as part of their deceit, they inform us of it so that we are astonished to discover our visions coming true. Indeed we get carried away with the notion that we are already close to the gift of foreknowledge.
To the credulous, a devil is a prophet; and to those who despise him, he is just a liar. Because he is a spiritual being, he knows what is happening in the lower regions, that someone is dying, for instance, so by way of dreams he passes the information on to the more gullible. However, demons lack actual foreknowledge. If they did not, these tricksters would be able to foretell our deaths.
Devils often take on the appearance of angels of light or martyrs and they appear to us in sleep and talk to us, so that they can push us into unholy joy and conceit when we wake up. But this very effect will reveal their trick, for what angels actually reveal are torments, judgments, and separation, with the result that on waking up we tremble and are miserable. And if we start to believe in the devils of our dreams, then we will be their playthings when we are also awake.
The man who believes in dreams shows his inexperience, while the man who distrusts every dream is very sensible. Trust only the dreams that foretell torments and judgment for you, but even these dreams may also be from demons if they produce despair in you.