Waking visions resemble dreams
Augustine is arguing that the dead do not know when the living see them, and he now extends the point from dreams to waking visions of the disturbed.
Some visions of waking people are also like dreams—people whose senses are disturbed, like the delirious or those raging in any kind of madness. They talk to themselves as if they were talking to people truly present, and they talk to absent people as if they were present, discerning their images whether living or dead. But just as the living do not know that they are being seen and spoken with by such people—for they are not really present or really conversing, but people with disturbed senses suffer such imaginary visions—so too those who have departed this life appear to people affected in this way as if they were present, when they are absent and completely unaware whether anyone sees them in an image.
Latin
Similia sunt autem somniis nonnulla etiam uisa uigilantium, qui turbatos habent sensus, sicut phrenetici uel quocumque furentes modo. nam et ipsi loquuntur secum, quasi uere praesentibus loquantur, <loquuntur> etiam cum absentibus quam praesentibus, quorum imagines cernunt siue uiuorum siue mortuorum. sed quemadmodum hi, qui uiuunt, ab eis se uideri et cum eis se colloqui nesciunt — neque enim re uera ipsi adsunt aut ipsi sermocinantur, sed turbatis sensibus homines talia uisa imaginaria patiuntur — eo modo et hi, qui ex hac uita migrarunt, sic affectis hominibus uidentur quasi praesentes, cum sint absentes et, utrum aliquis eos imaginaliter uideat, omnino nescientes.