It is evident that this principle is far too broad to be used for the purpose of affirming the impossibility of the action of external evil agents only. It is based on the supposition that our mental philosophy is so complete as to be able to assign even the most abnormal portions of our mental action to definite and known forces, all of which originate within the mind itself, and are never due to external influences. If mental philosophy could establish this as a fact, it would doubtless prove that possession was impossible; but it could prove a great deal more, even that God never acted on or influenced the spirit of man. But if there is any one phenomenon of the mind, of the origin of which we are ignorant, the whole principle is vitiated, for that very phenomenon may be caused by the action of an external power. The real point of the controversy therefore is, Is our mental science thus complete? Has it been able to reduce all our mental phenomena, including the most abnormal of them, to the action of known forces? Has it analyzed our mental powers to their inmost depths? Until it has done this, it is impossible to affirm that the abnormal actions of the mind may not be occasioned by an external agency. It will probably be urged, that although our philosophy has not yet succeeded in assigning all our mental phenomena to the action of known forces, it hopes to accomplish this hereafter; and that its past conquests ought to be accepted as a pledge of its future performances; and that the time will certainly come, when it will be able to refer every mental phenomenon to a cause originating in the mind itself, and acting in conformity with invariable law. Promises, however, are not performances; what is requisite to impart validity to wide affirmations is present actual knowledge, not the hope that future scientific conquests will be extended over the entire regions of the unknown. Science professes to walk by sight and not by faith. In a subject of this kind it is most unphilosophical to assume that the possibilities of the future are the realities of the present; and to enunciate propositions whose validity rests solely on the fact that they are so. I will now definitely state the principle which can alone give any scientific value to the assertion, that such demoniacal action as that which is described in the New Testament, is unbelievable. It is as follows: that we have so completely ascertained the nature of the forces which act on our minds, and the laws which regulate them, that we know as a scientifically established truth, that they all originate either in our own mental organization, or in the action of other men on our minds. The statement of the principle in this distinct form at once shows that it is invalid.