But the reason the devils turn themselves into Incubi or Succubi is not for the cause of pleasure, since a spirit has not flesh and blood; but chiefly it is with this intention, that through the vice of luxury they may work a twofold harm against men, that is, in body and in soul, that so men may be more given to all vices. And there is no doubt that they know under which stars the semen is most vigorous, and that men so conceived will be always perverted by witchcraft.
When Almighty God had enumerated many vices of luxury rife among the unbelievers and heretics, from which He wished His people to be clean, He says in Leviticus 18: Defile not ye yourselves in any of these things: for in all these the nations are defiled which I cast out before you: and the land is defiled: therefore I do visit the iniquity thereof upon it. The gloss explains the word “nations” as meaning devils who, on account of their multitude, are call the nations of the world, and rejoice in all sin, especially in fornication and idolatry, because by these are defiled the body and the soul, and the whole man, which is called “the land.” For every sin that a man commits is outside his body, but the man who commits fornication sins in his body. If anyone wishes to study further the histories concerning Incubi and Succubi, let him read (as has been said) Bede in his History of the English, and William, and finally Thomas of Brabant in his book About Bees.
To return to the matter in hand. And first for the natural act of propagation instituted by God, that is, between male and female; that as though by the permission of God the Sacrament of Matrimony can be made void by the work of the devil through witchcraft, as has been shown above. And the same is much more strongly true of any other venereal act between man and woman.
But if it is asked why the devil is allowed to cast spells upon the venereal act, rather than upon any other human act, it is answered that many reasons are assigned by the Doctors, which will be discussed later in the part concerning the divine permission. For the present that reason that has been mentioned before must suffice, namely, that the power of the devil lies in the privy parts of men. For of all struggles those are the harder where the fight is continuous and victory rare. And it is unsound to argue that in that case the work of the devil is stronger than the work of God, since the matrimonial act instituted by God can be made void: for the devil does not make it void by violence, since he has no power at all in the matter except as he is permitted by God. Therefore it would be better to argue from this that he is powerless.
Secondly, it is true that to procreate a man is the act of a living body. But when it is said that devils cannot give life, because that flows formally from the soul, it is true; but materially life springs from the semen, and an Incubus devil can, with God’s permission, accomplish this by coition. And the semen does not so much spring from him, as it is another man’s semen received by him for this purpose (see S. Thomas, I, 51, art. 3). For the devil is Succubus to a man, and becomes Incubus to a woman. In just the same way they absorb the seeds of other things for the generating of various things, as S. Augustine says, De Trinitate 3.
Now it may be asked, of whom is a child so born the son? It is clear that he is not the son of the devil, but of the man whose semen was received. But when it is urged that, just as in the works of Nature, so there is no superfluity in the works of angels, that is granted; but when it is inferred that the devil can receive and inject semen invisibly, this also is true; but he prefers to perform this visibly as a Succubus and an Incubus, that by such filthiness he may infect body and soul of all humanity, that is, of both woman and man, there being, as it were, actual bodily contact.
Moreover, devils can do invisibly more things which they are not permitted to do visibly, even if the so wished; but they are allowed to do them invisibly, either as a trial for the good, or as a punishment for the wicked. Finally, it may happen that another devil may take the place of the Succubus, receive the semen from him, and become and Incubus in the place of the other devil; and this for a threefold reason. Perhaps because one devil, allotted to a woman, should receive semen from another devil, allotted to a man, that in this way each of them should be commissioned by the prince of devils to work some witchcraft; since, to each one is allotted his own angel, even from among the evil ones; or because of the filthiness of the deed, which one devil would abhor to commit. For in many inquiries it is clearly shown that certain devils, out of some nobility in their natures, would shrink from
