Excerpted from Summa Theologiae 1.107, 111

But in addition to this way, the will in men can also be moved in another way from the outside, namely from the passion originating around the sensitive appetite, just as from desire or anger the will can be turned toward willing something. And so also the angels, insofar as they can arouse passions of this kind, can move the will. But this does not necessarily happen, because the will always remains free to agree with or resist a passion.

I answer that it must be said that angels, good as well as bad, can by the power of their own nature move the imagination of a man. Indeed, this can be understood as follows. For it was said above that corporeal nature obeys an angel in local motion. Therefore those things which can be caused by local motion of some bodies are within the natural power of angels. But it is clear that imaginary apparitions are sometimes caused in us by a local change of corporeal spirits and humors. Therefore Aristotle, in his On Sleep, assigning the cause of the apparitions of dreams, says that when an animal sleeps, because a lot of blood descends into the sensitive principle, at the same time motions descend, that is impressions left from the motions of the senses which are preserved in the sensual spirits, and they move the sensitive principle, in such a way that an apparition arises, just as if the sensitive principle were then being changed by the exterior things themselves. And such a great commotion of spirits and humors can occur that apparitions of this kind can happen even to those who are awake, such as happens to madmen and others of this kind.

Original Latin

Respondeo dicendum quod in Angelis est aliqua locutio, sed, sicut dicit Gregorius II Moral., dignum est ut mens nostra, qualitatem corporeae locutionis excedens, ad sublimes atque incognitos modos locutionis intimae suspendatur. Ad intelligendum igitur qualiter unus Angelus alii loquatur, considerandum est quod, sicut supra diximus cum de actibus et potentiis animae ageretur, voluntas movet intellectum ad suam operationem. Intelligibile autem est in intellectu tripliciter, primo quidem, habitualiter, vel secundum memoriam, ut Augustinus dicit; secundo autem, ut in actu consideratum vel conceptum; tertio, ut ad aliud relatum.

Manifestum est autem quod de primo gradu in secundum transfertur intelligibile per imperium voluntatis, unde in definitione habitus dicitur, quo quis utitur cum voluerit. Similiter autem et de secundo gradu transfertur in tertium per voluntatem, nam per voluntatem conceptus mentis ordinatur ad alterum, puta vel ad agendum aliquid, vel ad manifestandum alteri. Quando autem mens convertit se ad actu considerandum quod habet in habitu, loquitur aliquis sibi ipsi, nam ipse conceptus mentis interius verbum vocatur.

Ex hoc vero quod conceptus mentis angelicae ordinatur ad manifestandum alteri, per voluntatem ipsius Angeli, conceptus mentis unius Angeli innotescit alteri, et sic loquitur unus Angelus alteri. Nihil est enim aliud loqui ad alterum, quam conceptum mentis alteri manifestare.