Here follows the Way whereby Witches copulate with those Devils known as Incubi.
… we will speak first of the matter and quality of the body which the devil assumes.
It must be said that he assumes an aerial body, and that it is in some respects terrestrial, in so far as it has an earthly property through condensation; and this is explained as follows. The air cannot of itself take definite shape, except the shape of some other body in which it is included. And in that case it is not bound by its own limits, but by those of something else; and one part of the air continues into the next part. Therefore he cannot simply assume an aerial body as such.
Know, moreover, that the air is in every way a most changeable and fluid matter: and a sign of this is the fact that when any have tried to cut or pierce with a sword the body assumed by a devil, they have not been able to; for the divided parts of the air at once join together again. From this it follows that air is in itself a very competent matter, but because it cannot take shape unless some other terrestrial matter is joined with it, therefore it is necessary that the air which forms the devil’s assumed body should be in some way inspissated, and approach the property of the earth, while still retaining its true property as air. And devils and disembodied spirits can effect this condensation by means of gross vapours raised from the earth, and by collecting them together into shapes in which they abide, not as defilers of them, but only as their motive power which give to that body the formal appearance of life, in very much the same way as the soul informs the body to which it is joined. They are, moreover, in these assumed and shaped bodies like a sailor in a ship which the wind moves.
So when it is asked of what sort is the body assumed by the devil, it is to be said that with regard to its material, it is one thing to speak of the beginning of its assumption, and another thing to speak of its end. For in the beginning it is just air; but in the end it is inspisated air, partaking of some of the properties of the earth. And all this the devils, with God’s permission, can do of their own nature; for the spiritual nature is superior to the bodily. Therefore the bodily nature must obey the devils in respect of local motion, though not in respect of the assumption of natural shapes, either accidental or substantial, except in the case of some small creatures (and then only with the help of some other agent, as has been hinted before). But as to local motion, no shape is beyond their power; thus they can move them as they wish, in such circumstances as they will.
From this there may arise an incidental question as to what should be thought when a good or bad Angel performs some of the functions of life by means of true natural bodies, and not in aerial bodies; as in the case of Balaam’s ass, through which the Angel spoke, and when the devils take possession of bodies. It is to be said that those bodies are not called assumed, but occupied. See S. Thomas, II. 8, Whether Angels assume bodies. But let us keep strictly to our argument.
In what way is it to be understood that devils talk with witches, see them, hear them, eat with them, and copulate with them? And this is the second part of this first difficulty.
For the first, it is to be said that three things are required for true conversation: namely, lungs to draw in the air; and this is not only for the sake of producing sound, but also to cool the heart; and even mutes have this necessary quality.
Secondly, it is necessary that some percussion be made of a body in the air, as a greater or less sound is made when one beats wood in the airs, or rings a bell. For when a substance that is susceptible to sound is struck by a sound-producing instrument, it gives out a sound according to its size, which is received in the air and multiplied to the ears of the hearer, to whom, if he is far off, it seems to come through space.
Thirdly, a voice is required; and it may be said that what is called Sound in inanimate bodies is called Voice in living bodies. And here the tongue strikes the respirations of air against an instrument or living natural organ provided by God. And this is not a bell, which is called a sound, whereas this is a voice. And this third requisite may clearly be exemplified by the second; and I have set this down that preachers may have a method of teaching the people.
And fourthly, it is necessary that he who forms the voice should mean to express by means of that voice some concept of the mind to someone else, and that he should himself understand what he is saying; and so manage his voice by successively striking his teeth with his tongue in his mouth, by opening and shutting his lips, and by sending the air struck in his mouth into the outer air, that in this way the sound is reproduced in order in the ears of the hearer, who then understands his meaning.
To return to the point. Devils have no lungs or tongue, though they can show the latter, as well as teeth and lips, artificially made according to the condition of their body; therefore they cannot truly and properly speak. But since they have understanding, and when they wish to express their meaning, then, by some disturbance of the air included in their assumed body, not of air breathed in and out as in the case of men, they produce, not voices, but sounds which have some likeness to voices, and send them articulately through the outside air to the ears of the hearer. And that the likeness of a voice can be made without respiration of air is clear from the case of other animals which do not breathe, but are said to made a sound, as do also certain other instruments, as Aristotle says in the de Anima. For certain fishes, when they are caught, suddenly utter a cry outside the water, and die.
– Heinrich Kramer and James Sprenger, The Malleus Maleficarum