As to the second argument, that the devil cannot effect physical changes in the body; this is in part true, and in part not, and this is with reference to three sorts of mutation. For the devil cannot change the body in such a way that its whole shape and appearance is altered (which is rather to be called a new production than a change) without the help of some agent, or with the permission of God. But if we speak of a change in quality, as in the matter of sickness and health, as has been shown before, he can inflict upon the body various diseases, even to taking away the reason, and so can cause inordinate hatred and love.
And a third kind of mutation can be added, which is when a good or bad angel enters into the body, in the same way that we say that God alone is able to enter into the soul, that is, the essence of life. But when we speak of an angel, especially a bad angel, entering the body, as in the case of an obsession, he does not enter beyond the limits of the essence of the body; for in this way only God the Creator can enter, Who gave it to be as it were the intrinsic operation of life. But the devil is said to enter the body when he effects something about the body: for where he works, there he is, as S. John Damascene says. And then he works within the bounds of corporeal matter, but not within the very essence of the body.
From this it appears that the body has two properties, matter and spirit. And this is like the distinction between the apparent and the real. Therefore when devils enter the body, they enter the powers belonging to the bodily organs, and can so create impressions on those powers. And so it happens that through such operations and impressions a phantasm is projected before the understanding, such as the seeing of colours, as it is said in the 3rd book De Anima. And so this impression penetrates also to the will. For the will takes its conception of what is good from the intellect, according as the intellect accepts something as good either in truth or in appearance.
As for the third argument: a knowledge of the thoughts of the heart may come about in two ways, either from seeing their effects or by reading them actually in the intellect. In the first way they can be known not only by an angel, but even by man, although it will be shown that an angel has more skill in this matter. For sometimes the thoughts are made evident, not only by some external action, but even by a change in the countenance. And doctors also can discern some affections of the mind through the pulse. Wherefore S. Augustine says (De Diuinatione Daemonum) that sometimes it is very easy to tell a man’s disposition, not only from his words, but from his very thoughts, which are signs of the soul expressed in the body; although in his book of Retractations he says that no definite rule can be laid down how this can be done; and I think that he is reluctant to admit that the devil can know the inner thoughts of the heart.
From another point of view, the thoughts of the intellect and the affections of the will can be known only by God. For the will of a rational creature is subject only to God, and He alone can work in it Who is its first cause and ultimate end. Therefore that which is in the will, or depends only on the will, is known only to God. Moreover, it is manifest what depends only on the will, if one considers things by their resultant actions. For when a man has the quality of knowledge, and the understanding that comes from it, he uses it when he wills.
It is proved, then, from what has been said, that a spirit cannot enter the soul, therefore he cannot, naturally, see what is in the mind, especially what is in the inner depths of the soul. Wherefore, when it is argued that the devil cannot see the thoughts of the heart, and therefore cannot move the hearts of men to love or hatred, it is answered that he does learn men’s thoughts through their visible effects, and is more skilful in this matter than man; and so by subtle ways he can move men to love and hatred, by creating phantasms and darkening the intellect.
And this must be said by way of comfort to relieve the apprehensions of the virtuous: that when the sensible exterior and bodily change which accompanied men’s thoughts is so vague and indeterminate that the devil cannot by it arrive at any certain knowledge of the thoughts, especially when the virtuous at times take a little leisure from study and good works, he molests them then chiefly in dreams; as is known by experience. But when the physical effect of thought is strong and determinate, the devil can know by a man’s appearance whether his thoughts are turned towards envy or luxury. But we find that
