2. "There is nothing superstitious or unlawful in simply applying natural agencies to the production of certain effects, of which they are supposed to be naturally capable.... We must consider whether there is a fair appearance of the cause being able to produce the effect naturally. If there is, the experiment will not be unlawful: for it is lawful to use natural causes in order to their proper effects." (2a 2æ, q. 96, art. 2, in corp., ad 1.) But this we must understand under two provisos. First, that the "fair appearance" spoken of be not opposed by a considerable force of evidence, whether of authority or of reason, tending the other way: for in this matter, which is not a mere matter of legality, it is not permissible to run risks of becoming familiar with God's enemies. Secondly, that the cause, though natural, be not morally prejudicial. Not even a natural cause, brandy for instance, may be used to all its effects. Thus for the mesmeric sleep, though that should be proved to be purely natural, yet the weakening of the will thence ensuing, and the almost irresistible dominion acquired by the operator over his patient, render it imperative that such a remedy should not be applied without grave necessity, and under an operator of assured moral character. 3. St. Thomas continues in the place last quoted: "Wherefore, if there is no fair appearance of the causes employed being able to produce such effects, it needs must be that they are not employed to the causation of these effects as causes, but only as signs, and thus they come under the category of preconcerted signals arranged with evil spirits." The modern Spiritualist is only too forward to avow his understanding with the unseen powers; but he will have it that the spirits that he deals with are good and harmless. We must prove the spirits by the general effects of their communications--whether they be in accordance with the known laws of morality, and the assured teachings of religion, natural and revealed. Also we must consider, from what we know from approved sources concerning God, and His holy angels, and the spirits of the just, either already made perfect, or still suffering for a time, whether they are likely to respond to such signs as Spiritualists commonly employ. Also we must not ignore, what revelation tells us, of an "enemy," a "father of lies," who "changes himself into an angel of light," and who is ever ready, so far as it is permitted him, to eke out curiosity, folly, and credulity, such as he found in Eve. _Readings_.--St. Thos., 2a 2æ, q. 93; _ib_., q. 95, art. 4, in corp. SECTION III.--_Of the duty of knowing God_.