I lay down as a principle that to explain the affair of apparitions, and to give on this subject any certain rules, we should--
1st. Know perfectly the nature of spirits, angels and souls, and demons. We should know whether souls by nature are so spiritualized that they have no longer any relation to matter; or if they have, again, any alliance with an aërial, subtile, invisible body, which they still govern after death; or whether they exert any power over the body they once animated, to impel it to certain movements, as the soul which animates us gives to our bodies such impulsions as she thinks proper; or whether the soul determines simply by its will, as occasional or secondary cause, the first cause, which is God, to put in motion the machine which it once animated.
2d. If after death the soul still retains that power over its own body, or over others; for instance, over the air and other elements.
3d. If angels and demons have respectively the same power over sublunary bodies--for instance, to thicken air, inflame it, produce in it clouds and storms; to make phantoms appear in it; to spoil or preserve fruits and crops; to cause animals to perish, produce maladies, excite tempests and shipwrecks at sea; or even to fascinate the eyes and deceive the other senses.
4th. If they can do all these things naturally, and by their own virtue, as often as they think proper; or if there must be a particular order, or at least permission from God, for them to do what we have just said.
5th. Lastly, we should know exactly what power is possessed by these substances which we suppose to be purely spiritual, and how far the power of the angels, demons, and souls separated from their gross bodies, extends, in regard to the apparitions, operations and movements attributed to them. For whilst we are ignorant of the power which the Creator has given or left to disembodied souls, or to demons, we can in no way define what is miraculous, or prescribe the just bound to which may extend, or within which may be limited, the natural operations of spirits, angels, and demons.
If we accord the demon the faculty of fascinating our eyes when it pleases him, or of disposing the air so as to form the appearance of a phantom, or phenomenon; or of restoring movement to a body which is dead but not entirely corrupted; or of disturbing the living by ill dreams, or terrific representations, we should no longer admire many things which we admire at present, nor regard as miracles certain cures and certain apparitions, if they are only the natural effects of the power of souls, angels and demons.