I own it all; but I dare maintain that that is not possible either to angel or demon, nor to any spiritual substance whatsoever. The soul can produce in herself thoughts, will, and wishes; she can give her impulsion to the movements of her body, and repress its sallies and agitations; but how does she do that? Philosophy can hardly explain it, but by saying that by virtue of the union between herself and the body, God, by an effect of his wisdom, has given her power to act upon the humors, its organs, and impress them with certain movements; but there is reason to believe that the soul performs all that only as an occasional cause, and that it is God as the first, necessary, immediate, and essential cause, which produces all the movements of the body that are made in a natural way.
Neither angel nor demon has more privilege in this respect over matter than the soul of man has over its own body. They can neither modify matter, change it, nor impress it with action and motion, save by the power of God, and with his concurrence both necessary and immediate; our knowledge does not permit us to judge otherwise; there is no physical proportion between the spirit and the body; those two substances cannot act mutually and immediately one upon the other; they can act only occasionally, by determining the first cause, in virtue of the laws which wisdom has judged it proper to prescribe to herself for the reciprocal action of the creatures upon each other, to give them being, to preserve it, and perpetuate movement in the mass of matter which composes the universe, in himself giving life to spiritual substances, and permitting them with his concurrence, as the First Cause, to act, the body on the soul, and the soul on the body, one on the other, as secondary causes.
Porphyry, when consulted by Anebo, an Egyptian priest, if those who foretell the future and perform prodigies have more powerful souls, or whether they receive power from some strange spirit, replies that, according to appearance, all these things are done by means of certain evil spirits that are naturally knavish, and take all sorts of shapes, and do everything that one sees happen, whether good or evil; but that in the end they never lead men to what is truly good.