But it is not to be thought that this is the only way he influences a man; for in that case no sin would proceed from the devil’s instruction, except such as were suggested by him in visible form. Therefore it must be said that even invisibly he instigates man to sin. And this he does in two ways, either by persuasion or by disposition. By persuasion, he presents something to the understanding as being a good thing. And this he can do in three ways; for he presents it either to the intellect, or to the inner perceptions, or to the outer. And as for the intellect; the human intellect can be helped by a good Angel to understand a thing by means of enlightenment, as Dionysius says; and to understand a thing, according to Aristotle, is to suffer something: therefore the devil can impress some form upon the intellect, by which the act of understanding is called forth.
And it may be argued that the devil can do this by his natural power, which is not, as had been shown, diminished. It is to be said, however, that he cannot do this by means of enlightenment, but by persuasion. For the intellect of man is of that condition that, the more it is enlightened, the more it knows the truth, and the more it can defend itself from deception. And because the devil intends his deception to be permanent, therefore no persuasion that he uses can be called enlightenment: although it may be called revelation, in that when he invisibly uses persuasion, by means of some impression he plants something on the inner or outer sense. And by this the reasoning intellect is persuaded to perform some action.
But as to how he is enabled to create an impression on the inner sense, it is to be noted that the bodily nature is naturally born to be moved locally by the spiritual; which is clear from the case of our own bodies, which are moved by souls; and the same is the case with the stars. But it is not by nature adapted to be directly subject to influences, by which we mean outside influences, not those with which it is informed. Wherefore the concurrence of some bodily agent is necessary, as is proved in the 7th book of the Metaphysics. Corporeal matter naturally obeys a good or bad angel as to the local motion; and it is due to this that devils can through motion collect semen, and employ it for the production of wonderful results. This was how it happened that Pharaoh’s magicians produced serpents and actual animals, when corresponding active and passive agents were brought together. Therefore there is nothing to prevent the devils from effecting anything that appertains to the local motion of corporeal matter, unless God prevent it.
And now let us examine how the devil can through local motion excite the fancy and inner sensory perceptions of a man by apparitions and impulsive actions. It is to be noted that Aristotle (De Somno et Uigilia) assigns the cause of apparitions in dreams through local motion to the fact that, when an animal sleeps the blood flows to the inmost seat of the senses, from which descend motions or impressions which remain from past impressions preserved in the mind or inner perception; and these are Fancy or Imagination, which are the same thing according to S. Thomas, as will be shown.
For fancy or imagination is as it were the treasury of ideas received through the senses. And through this it happens that devils stir up the inner perceptions, that is the power of conserving images, that they appear to be a new impression at that moment received from exterior things.
It is true that all do not agree to this; but if anyone wishes to occupy himself with this question, he must consider the number and the office of the inner perceptions. According to Avicenna, in his book On the Mind, these are five: namely, Common Sense, Fancy, Imagination, Thought, and Memory. But S. Thomas, in the First Part of Question 79, says that they are only four, since Fancy and Imagination are the same thing. For fear of prolixity I omit much more that has variously been said on this subject.
Only this must be said; that fancy is the treasury of ideas, but memory appears to be something different. For fancy is the treasury or repository of ideas received through the senses; but memory is the treasury of instincts, which are not received through the senses. For when a man sees a wolf, he runs away, not because of its ugly colour or appearance, which are ideas received through the outer senses and conserved in his fancy; but he runs away because the wolf is his natural enemy. And this he knows through some instinct or fear, which is apart from thought, which recognized the wolf as hostile, but a dog as friendly. But the repository of those instincts is memory. And reception and retention are two different things in animal nature; for those who are of a humid disposition receive readily, but retain badly; and the contrary is the case of those with a dry humour.
To return to the question. The apparitions that come in dreams to sleepers proceed from the ideas retained in the repository of their mind, through a natural local motion caused by the flow of blood to the first and inmost seat of their faculties of perception; and we speak of an instrinsic local motion in the head and the cells of the brain.
And this can also happen through a
