Oneiromancy may be practised in two ways. The first is when a person uses dreams so that he may dip into the occult with the help of the revelation of devils invoked by him, with whom he has entered into an open pact. The second is when a man uses dreams for knowing the future, in so far as there is such virtue in dreams proceeding from Divine revelation, from a natural and instinsic or extrinsic cause; and such divination would not be unlawful. So says S. Thomas.
And that preachers may have at least the nucleus of an understanding of this matter, we must first speak about the Angels. An Angel is of limited power, and can more effectively reveal the future when the mind is adapted to such revelations than when it is not. Now the mind is chiefly so adapted after the relaxation of exterior and interior movement, as when nights are silent and the fumes of motion are quieted; and these conditions are fulfilled round about the dawn, when digestion is completed. And I say this of us who are sinners, to whom the Angels in their Divine piety, and in the execution of their offices, reveal certain things, so that when we study at the time of the dawn we are given an understanding of certain occult matters in the Scriptures. For a good Angel presides over our understanding, just as God does over our will, and the stars over our bodies. But to certain more perfect men the Angel can at any hour reveal things, whether they are awake or asleep. However, according to Aristotle, De Somno et Uigilia, such men are more apt to receive revelations at one time than at another; and this is the case in all matters of magic.
Secondly, it is to be noted that it happens through Nature’s care for and regulation of the body, that certain future events have their natural cause in a man’s dreams. And then those dreams or visions are not causes, as was said in the case of Angels, but only signs of that which is coming to a man in the future, such as health or sickness or danger. And this is the opinion of Aristotle. For in the dreams of the spirit Nature images the disposition of the heart, by which sickness or some other thing naturally comes to a man in the future. For if a man dreams of fires, it is a sign of a choleric disposition; if of flying or some such thing, it is a sign of a sanguine disposition; if he dreams of water or some other liquid, it is a sign of a phlegmatic, and if he dreams of terrene matters, it is a sign of a melancholy disposition. And therefore doctors are very often helped by dreams in their diagnosis (as Aristotle says in the same book).
But these are slight matters in comparison with the unholy dreams of witches. For when they do not wish, as has been mentioned above, to be bodily transferred to a place, but desire to see what their fellow-witches are doing, it is their practice to lie down on their left side in the name of their own and of all devils; and these things are revealed to their vision in images. And if they seek to know some secret, either for themselves of for others, they learn it in dreams from the devil, by reason of an open, not a tacit, pact entered into with him. And this pact, again, is not a symbolical one, accomplished by the sacrifice of some animal, or some act of sacrilege, or by embracing the worship of some strange cult; but it is an actual offering of themselves, body and soul, to the devil, by a sacrilegiously uttered and inwardly purposed abnegation of the Faith. And not content with this, they even kill, or offer to devils, their own and others’ children.
Another species of divination is practised by Pythons, so called from Pythian Apollo, who is said to have been the originator of this kind of divination, according to S. Isidore. This is not effected by dreams or by converse with the dead, but by means of living men, as in the case of those who are lashed into a frenzy by the devil, either willingly or unwillingly, only for the purpose of foretelling the future, and not for the perpetration of any
