The way demons sin is constrained, and has to do with who they would follow.
1 However, that there is sin of the will in demons is obvious from the text of Sacred Scripture. In fact, it is said in 1 John (3:8) that “the devil sins from the beginning”; and in John (8:44) it is said that “the devil is a liar and the father of lies” and that “he was a murderer from the beginning.” And in Wisdom (2:24) it is said that “by the envy of the devil, death came into the world.”
2 Moreover, if anyone wished to follow the views of the Platonists, that would be an easy way to answer the arguments stated above. For they say that demons are animals with an aerial body; and so, since they have bodies united to them, there can also be in them a sensitive part. Hence, they also attribute passions to them, which are for us a cause of sin; namely, anger, hate, and others of like kind. This is why Apuleius says that they are passive in their mind.
3 Also, apart from this contention that they are united to bodies according to the views of Plato, it might perhaps be possible to claim another kind of knowledge in them, other than that of the intellect. For, according to Plato, the sensitive soul is also incorruptible. Hence, it must have an operation in which the body does not share. Thus, nothing is to prevent the operation of the sensitive soul and, consequently, passions from taking place in any intellectual substance, even though it is not united with a body. And so, there remains in them the same source of sinful action that is found in us.
4 However, both of these foregoing views are impossible. As a matter of fact, we showed above that there are no other intellectual substances united to bodies besides human souls. Moreover, that the operations of the sensitive soul cannot go on without the body is apparent from the fact that, with the corruption of any organ of sensation, the operation of one sense is corrupted. For instance, if the eye be destroyed, vision fails. For this reason, when the organ of touch is corrupted, without which an animal cannot exist, the animal must die.
5 So, for the clarification of the aforesaid difficulty, we must give some consideration to the fact that, as there is an order in agent causes, so also is there one in final causes, so that, for instance, a secondary end depends on a principal one, just as a secondary agent depends on a principal one.
Now, something wrong happens in the case of agent causes when a secondary agent departs from the order of the principal agent. For example, when the leg bone fails because of its crookedness in the carrying out of the motion which the appetitive power has commanded, limping ensues. So, too, in the case of final causes, when a secondary end is not included under the order of the principal end, there results a sin of the will, whose object is the good and the end.
6 Now, every will naturally wishes what is a proper good for the volitional agent, namely, perfect being itself, and it cannot will the contrary of this. So, in the case of a volitional agent whose proper good is the ultimate end, no sin of the will can occur, for the ultimate end is not included under the order of another end; instead, all other ends are contained under its order. Now, this kind of volitional agent is God, Whose being is the highest goodness, which is the ultimate end. Hence, in God there can be no sin of the will.
Notes Hence, for us, the importance of learning our proper ends.
7 But in any other kind of volitional agent, whose proper good must be included under the order of another good, it is possible for sin of the will to occur, if it be considered in its own nature. Indeed, although natural inclination of the will is present in every volitional agent to will and to love its own perfection so that it cannot will the contrary of this, yet it is not so naturally implanted in the agent to so order its perfection to another end, that it cannot fail in regard to it, for the higher end is not proper to its nature, but to a higher nature.
It is left, then, to the agent’s choice, to order his own proper perfection to a higher end. In fact, this is the difference between those agents who have a will, and those things which are devoid of will: the possessors of will order themselves and their actions to the end, and so they are said to be free in their choice; whereas those devoid of will do not order themselves to their end, but are ordered by a higher agent, being moved by another being to the end, not by themselves.
Notes You cannot escape free will.
8 Therefore, it was possible for sin to occur in the will of a separate substance, because it did not order its proper good and perfection to its ultimate end, but stuck to its own good as an end. And because the rules of action must be derived from the end, the consequence is that this separate substance tried to arrange for the regulation of other beings from himself wherein he had established his end, and thus his will was not regulated by another, higher one.
But this function belongs to God alone. In terms of this, we should understand that “he desired to be equal to God” (Is. 14:14). Not, indeed, that his good would be equal to the divine good, for this thought could not have occurred in his understanding, and in desiring such a thing he would have desired not to exist, since the distinction of species arises from the different grades of things, as is clear from previous statements.
However, to will to rule others, and not to have his will ruled by a higher one, is to will to take first place and, in a sense, not to be submissive; this is the sin of pride. Hence, it may appropriately be said that the first sin of the demon was pride. But since a diversified and pluralized error results from one error concerning the starting point, multiple sin followed in his will as a result of the first disorder of the will which took place in the demon: sins both of hatred toward God, as One Who resists his pride and punishes his fault most justly, and of envy toward man, and many other similar sins.
Notes It is no coincidence that “Pride” parades are thus demonic.
9 We should also consider that, when an agent’s proper good is related to several higher goods, the volitional agent is free to depart from the order of one superior and free not to abandon the order of another, whether it be higher or lower. Thus, a soldier who is subordinate to the king and to the leader of the army can order his will to the leader’s good and not to the king’s, or vice versa.
But, if the leader departs from the order of the king, the will of the soldier who abandons the will of the leader and directs his will to the king is going to be good, whereas the will of the soldier who follows the will of the leader against the will of the king is going to be bad, for the order of a lower principle depends on the order of a higher one.
Now, separate substances are not only subordinated to God, but one of them is subordinated to another, from the first to the last, as we showed in Book Two. And since in each volitional agent under God there can be a sin of the will, if he were considered in his own nature, it was possible for some of the higher ones, or even the highest of all, to sin in his will.
And, in fact, this is probably what happened, for he would not have been satisfied with his own good as an end unless his good were quite perfect. So, it possibly happened in this way: some of the lower ones, through their own will, ordered their good to his, and departing from the divine order they sinned in like fashion; others, however, observing in the movement of their will the divine order, rightly departed from the order of the sinful one, even though he was a superior in the order of nature. But how the will of both kinds perseveres immutably in goodness, or in evil, will be shown in Book Four, for this has to do with the punishments and rewards of the good and the evil.
Notes Consequently, if the king departs from the order of God, the will of soldier who abandons the will God and follows the will of the king is going to be bad.
10 But there is this difference between man and a separate substance: in one man there are several appetitive powers, one subordinated to the other. Now, this is not the case in separate substances, though one of these substances stands under another.
Now, sin occurs in the will when in any way the lower appetite rebels. So, just as sin could occur in the separate substances, either by being turned away from the divine order, or by one of the lower ones being turned aside from the order of a superior one which continues under the divine order, so also, in one man, sin may occur in two ways.
One way is due to the fact that the human will does not order its proper good to God; in fact, this kind of sin is common both to man himself and to the separate substance. Another way is due to the good of the lower appetite not being ruled in accord with the higher appetite; for example, we may desire the pleasures of the flesh to which the concupiscible appetite inclines, in discord with the order of reason. Now, this latter kind of sin cannot occur in separate substances.