But further: while madness is produced by physical causes, it is a certain fact that it is frequently occasioned by causes purely mental. Of this the instances are innumerable. These mental causes react on the brain and the nervous system; and thus they superinduce disease on those parts of our bodily organization by means of which the mind exercises its powers. Still the disease itself originates in causes that are not seated in the body, but in the mind. The mind is therefore capable of acting powerfully on our bodily frame. If therefore possession be viewed as the action of one mind on another, there is no reason why it should not be able to superinduce those forms of bodily derangement which exhibited themselves in the demoniacs by the simple action of the mind upon the body. The mental causes capable of producing mania are, as we know, of a varied description; and among them is the action and influence which one mind is capable of exerting on another. As, therefore, in certain states of our minds, or of our nervous system, mania with all its results can be produced by the simple action of mind on mind, and through the action of the mind disorder may be produced in our bodily organization, there can be no reason why possession with all its attendant phenomena should not originate in similar causes. There is nothing to imply that the superhuman agency manifested in possession was directly exerted on the body of the possessed. An agency which was entirely mental was fully adequate to produce all the phenomena with which it was accompanied. In cases of mania produced by mental action the removal of the exciting cause is the precondition of its cure, and in many cases effects it. Similarly, in cases of possession the removal of the exciting cause would produce similar results. It follows, therefore, from the foregoing considerations, that the allegation that the possessions described in the New Testament are incredible, because they contradict the known truths of mental science, is disproved. The question really resolves itself into the following one: Do evil beings, other than men, exist in the universe? Or, if they exist, is it credible that they are allowed to interfere in the affairs of men? This question we have already considered in a former chapter, and we have arrived at the conclusion that if we free ourselves from the trammels of _à priori_ theories, and judge only by the facts of the universe as it exists, neither their existence nor their intervention in human affairs is contrary to our reason.