How demons predict what they themselves will do or read from signs
Augustine is explaining the natural limits and methods of demonic divination, after arguing that demons' bodily abilities do not make them superior to good humans.
Since this is so, the first thing to understand—because the question is about the divination of demons—is that they usually foretell what they themselves are going to do. They often receive power to send diseases, to make the air itself unhealthy by corrupting it, and to persuade wicked deeds to perverse lovers of earthly advantages, men whose character they know for certain will consent to such persuasion. They persuade in astonishing and invisible ways, penetrating the bodies of unaware people through the subtlety of their own bodies and mixing themselves into their thoughts through certain imaginary visions, whether they are awake or asleep.
Sometimes, however, they predict not what they themselves will do but what they know beforehand from natural signs—signs that cannot reach human senses. A physician foresees what someone ignorant of his art does not foresee, but that does not make him a diviner. What is remarkable if, just as the physician foresees future good or bad health in the disturbed or balanced condition of the human body, so the demon foresees future storms in the state and ordering of the air, which is known to him and unknown to us?
Sometimes they also learn with complete ease the dispositions of human beings—not only those spoken aloud but even those conceived in thought—since certain signs from the mind are expressed in the body, and from this they foretell many future things that seem astonishing to others who do not know these dispositions. Just as a more agitated movement of the mind appears in the face, so that even other people recognize outwardly what is happening inwardly, so it should not be incredible that even gentler thoughts give some signs through the body, signs that cannot be known by the dull senses of human beings but can be known by the keen senses of demons.
Latin
Quae cum ita sint, primum sciendum est, quoniam de diuinatione daemonum quaestio est, illos ea plerumque praenuntiare, quae ipsi facturi sunt. accipiunt enim saepe potestatem et morbos inmittere et ipsum aerem uitiando morbidum reddere et peruersis atque amatoribus terrenorum commodorum malefacta suadere, de quorum moribus certi sunt, quod sint eis talia suadentibus consensuri. suadent autem miris et inuisibilibus modis per illam subtilitatem corporum suorum corpora hominum non sentientium penetrando et se cogitationibus eorum per quaedam imaginaria uisa miscendo, siue uigilantium siue dormientium. aliquando autem non quae ipsi faciunt, sed quae naturalibus signis futura praenoscunt, quae signa in hominum sensus uenire non possunt, ante praedicunt. neque enim quia praeuidet medicus, quod non praeuidet eius artis ignarus, ideo iam diuinus habendus est. quid autem mirum, si quemadmodum ille in corporis humani uel perturbata uel modificata temperie seu bonas seu malas futuras praeuidet ualitudines, sic daemon in aeris affectione atque ordinatione sibi nota, nobis ignota futuras praeuidet tempestates'? aliquando et hominum dispositiones non solum uoce prolatas, uerum etiam cogitatione conceptas, cura signa quaedam ex animo exprimuntur in corpore, tota facilitate perdiscunt atque hinc etiam multa futura praenuntiant, aliis uidelicet mira, qui ista disposita non nouerunt. sicut enim adparet concitatior animi motus in uultu, ut ab hominibus quoque aliquid forinsecus agnoscatur, quod intrinsecus agitur, ita non debet esse incredibile, si etiam leniores cogitationes dant aliqua signa per corpus, quae obtuso sensu hominum cognosci non possunt, acuto autem daemonum possunt.