One fact which we must always bear in mind is: man possesses free will. Godís intention is for man to control himself. Man has the authority to regulate his every natural endowment; hence his mental processes should be subject to the I power of his will. A Christian ought to inquire of himself: Are these my thoughts? Is it I who am thinking? If it is not I thinking, it must then be the evil spirit who is able to work in manís mind. Since I will not to think (and my mind usually follows my will) then the thoughts which presently arise in my head cannot be mine but rather are those which emanate from another ďpersonĒ who uses the ability of my mind against my own will. The person should know that in case he has not intended to think and yet there are thoughts arising in his head, he must conclude that these are not of him but of the evil spirit.
To determine whether an idea is of himself or of the wicked spirit, a Christian should observe how it arose. If in the beginning his mental faculty is peaceful and composed and is functioning normally and naturally according to the circumstances he is in, but suddenly a thought or a complete idea (having no bearing on his present circumstances or the work in which he is engaged) flashes across his brain, such inordinate and lightning quick thought is most likely the action of evil spirits. They are attempting to inject their thoughts into the believerís head and thus induce him to accept them as his own. It is unmistakable that the notion which the evil spirits introduce into manís mind is a matter he has not thought about at the moment and which does not follow the trend of his thinking. It is entirely ďnewĒósomething he never thought of himself. It has arisen abruptly and all by itself. When one gains this kind of thought it is well for him to inquire: Do I really think in this manner? Is it really I who am thinking? Do I want to think that way? Or is this something which simply becomes activated in my mind all by itself? The child of God should determine whether or not it is he himself who does the thinking. If he has not originated the idea but on the contrary opposses it, and yet it abides in his head, he then can assume that that idea issues from the enemy. Each thought which man chooses not to think and each one which opposes manís will come not from the man but from the outside.
Oftentimes also oneís brain is abounding with sundry ideas which he is helpless to stop. His head is like a thought machine, operated by external force; it continues to think but is impotent to desist. The believer may shake his head repeatedly, yet he cannot shake off the thoughts in his mind. They come to him in waves, rolling unceasingly day and night. There is no way to terminate them. He is not aware that this is but the activity of the evil spirit. He ought to understand what a ďthoughtĒ is. It is something which his mind grasps at. But in the case of these unmanageable thoughts it is not that his mind is grasping at something but rather that something is grasping his mind. In the natural course of events it is the mind which thinks about matters; now it is these matters which force the mind to think. Frequently a person wishes to set aside a matter but some external power keeps reminding him of it, not permitting him to forget and forcing him to think on further. This is the perpetration of evil spirits.
To summarize, then, we should investigate every abnormal sign. Aside from a natural cause such as sickness, all other abnormal indicators have their source with evil spirits. God never interferes with the operation of manís natural ability; He never abruptly mixes in His thought with manís nor does He abruptly restrict or destroy the functioning of manís intellect. The lightning cessation of all thoughts as though the brain has become a vacuum, the flashing interjection of thought at complete variance with the trend then current in the mind, the hasty severance of memory as if a wire had snapped leaving the mind paralyzed: all these are the results of the operation of the enemy. Because the evil spirit has seized hold of the organ of thought, he is able either to force it to cease functioning or by loosening his grip to let it work again. We must recognize that natural causes can produce only natural symptoms. Flash thoughts or loss of memory are entirely beyond the ability or control of our will and are contrary to natural cause and effect: they must therefore be inspired by supernatural evil forces.
In his letter to the Ephesians Paul is found writing about ďthe spirit that is now at work in the sons of disobedienceĒ (2.2). It is very important to know that the powers of darkness work not only outside but inside the man as well. When men work they can at most do so with their words, gestures or bodily movements; the evil spirits, however, can work with all these but even more. They can act from the outside in the same way as man acts, but they can work additionally from the inside. This means that they can squeeze themselves into manís thought life and work therein. Man is not capable of doing this: he is unable to enter another manís brain, subtly making many suggestions and confusing this matter of the source of the thoughts; but the evil spirits can. They possess an ability in communication which man does not have. They work initially in manís mind and then reach to his emotion, for mind and emotion are closely knit: they operate first in the mind and from there they arrive at manís volition, because mind and will are intimately joined too. The manner by which these enemy spirits operate is to plant covertly in manís head notions which they enjoy so as to accomplish their aim, or, conversely, they block thoughts which they do not relish so that man cannot think them through. The Bible distinctly indicates that the powers of darkness are able both to impart ideas to man and to steal them from him. ďThe devil had already put it into the heart of Judas Iscariot, Simonís son, to betray himĒ (John 13.2). his shows that Satan can put his thought into manís mind. ďThen the devil comes and takes away the word from their heartsĒ (Luke 8.12). This attests that Satan removes whatever word man ought to remember and causes him to forget everything. These two verses reveal the two-fold operation of the evil spirits upon the mind of man, either to add something to, or subtract something from, his mind.