Thomas Aquinas

“ Further, the angel, as above explained, enlightens the human intellect by means of the phantasms. But as the imagination which serves the intellect can be changed by an angel, so can the sensitive appetite which serves the will, because it also is a faculty using a corporeal organ. Therefore as the angel enlightens the mind, so can he change the will. ”

“ Natural reason, which is immediately from God, can be strengthened by an angel, as we have said above. Again, the more the human intellect is strengthened, so much higher an intelligible truth can be elicited from the species derived from creatures. Thus man is assisted by an angel so that he may obtain from creatures a more perfect knowledge of God. ”

“ We now consider the action of the angels on man, and inquire: (1) How far they can change them by their own natural power; (2) How they are sent by God to the ministry of men; (3) How they guard and protect men. ”

“ The demon cannot put thoughts in our minds by causing them from within, since the act of the cogitative faculty is subject to the will; nevertheless the devil is called the kindler of thoughts, inasmuch as he incites to thought, by the desire of the things thought of, by way of persuasion, or by rousing the passions. ”

“ The human intellect in its present state can understand only by turning to the phantasms; but the human will can will something following the judgment of reason rather than the passion of the sensitive appetite. Hence the comparison does not hold.
Art. 3 - Whether an angel can change man's imagination? Objection 1: It would seem that an angel cannot change man's imagination. ”

“ Hence Aristotle says (De Somn. et Vigil.) [*De Insomniis iii.] , when assigning the cause of visions in dreams, that "when an animal sleeps, the blood descends in abundance to the sensitive principle, and movements descend with it," that is, the impressions left from the movements are preserved in the animal spirits, "and move the sensitive principle"; so that a certain appearance ensues, as if the sensitive principle were being then changed by the external objects themselves. ”