Objection 1. It would seem that divination by dreams is not unlawful. It is not unlawful to make use of divine instruction. Now men are instructed by God in dreams, for it is written (Job 33:15-16): "By a dream in a vision by night, when deep sleep falleth upon men, and they are sleeping in their beds, then He," God to wit, "openeth the ears of men, and teaching instructeth them in what they are to learn." Therefore it is not unlawful to make use of divination by dreams.
Objection 2. Further, those who interpret dreams, properly speaking, make use of divination by dreams. Now we read of holy men interpreting dreams: thus Joseph interpreted the dreams of Pharaoh's butler and of his chief baker (Genesis 40), and Daniel interpreted the dream of the king of Babylon (Daniel 2 and 4). Therefore divination by dreams is not unlawful.
Objection 3. Further, it is unreasonable to deny the common experiences of men. Now it is the experience of all that dreams are significative of the future. Therefore it is useless to deny the efficacy of dreams for the purpose of divination, and it is lawful to listen to them.
On the contrary, It is written (Deuteronomy 18:10): "Neither let there be found among you any one that... observeth dreams."
I answer that, As stated above (A2,6), divination is superstitious and unlawful when it is based on a false opinion. Wherefore we must consider what is true in the matter of foreknowing the future from dreams. Now dreams are sometimes the cause of future occurrences; for instance, when a person's mind becomes anxious through what it has seen in a dream and is thereby led to do something or avoid something: while sometimes dreams are signs of future happenings, in so far as they are referable to some common cause of both dreams and future occurrences, and in this way the future is frequently known from dreams. We must, then, consider what is the cause of dreams, and whether it can be the cause of future occurrences, or be cognizant of them.
Accordingly it is to be observed that the cause of dreams is sometimes in us and sometimes outside us. The inward cause of dreams is twofold: one regards the soul, in so far as those things which have occupied a man's thoughts and affections while awake recur to his imagination while asleep. A such like cause of dreams is not a cause of future occurrences, so that dreams of this kind are related accidentally to future occurrences, and if at any time they concur it will be by chance. But sometimes the inward cause of dreams regards the body: because the inward disposition of the body leads to the formation of a movement in the imagination consistent with that disposition; thus a man in whom there is abundance of cold humors dreams that he is in the water or snow: and for this reason physicians say that we should take note of dreams in order to discover internal dispositions.
In like manner the outward cause of dreams is twofold, corporal and spiritual. It is corporal in so far as the sleeper's imagination is affected either by the surrounding air, or through an impression of a heavenly body, so that certain images appear to the sleeper, in keeping with the disposition of the heavenly bodies. The spiritual cause is sometimes referable to God, Who reveals certain things to men in their dreams by the ministry of the angels, according Numbers 12:6, "If there be among you a prophet of the Lord, I will appear to him in a vision, or I will speak to him in a dream." Sometimes, however, it is due to the action of the demons that certain images appear to persons in their sleep, and by this means they, at times, reveal certain future things to those who have entered into an unlawful compact with them.
Accordingly we must say that there is no unlawful divination in making use of dreams for the foreknowledge of the future, so long as those dreams are due to divine revelation, or to some natural cause inward or outward, and so far as the efficacy of that cause extends. But it will be an unlawful and superstitious divination if it be caused by a revelation of the demons, with whom a compact has been made, whether explicit, through their being invoked for the purpose, or implicit, through the divination extending beyond its possible limits.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)