The rich man's request does not prove the dead know the living
Augustine is answering an objection from the parable of the rich man and Lazarus, arguing that the rich man cared about his living brothers without knowing what they were doing.
Someone may say: if the dead have no concern for the living, how is it that the rich man who was being tormented in the underworld asked father Abraham to send Lazarus to his five brothers who were not yet dead, to warn them, so that they would not also come into that same place of torments? But surely it does not follow that because that rich man said this, he knew what his brothers were doing or suffering at that time. He had concern for the living even though he had no idea what they were doing, just as we have concern for the dead even though we certainly do not know what they are doing. For if we had no concern for the dead, we would not pray to God for them. In the end Abraham did not send Lazarus, and he answered that they had Moses and the prophets here, whom they ought to obey so that they would not come to those punishments (Luke 16:29). Here again the question arises: how did father Abraham himself not know what was happening here, when he knew that Moses and the prophets were here—that is, their books, by obeying which people avoid the torments of the underworld—and when he also knew that the rich man had lived in pleasures while poor Lazarus had lived in labors and pains? For he says to him: “Remember, child, that you received good things in your life, and Lazarus in like manner evil things” (Luke 16:25). So he knew these things, which had certainly taken place among the living, not among the dead. But he could not have known them while they were happening among the living; he was able to learn them after they had died, with Lazarus indicating them—so that what the prophet says is not false: “Abraham did not know us” (Isaiah 63:16).
Latin
Dixerit aliquis: si nulla est mortuis cura de uiuis, quomodo ille diues, qui apud inferos torquebatur, rogabat Abraham patrem, ut mittat Lazarum ad quinque fratres suos nondum mortuos et agat cum eis, ne ueniant et ipsi in eundem tormentorum locum? sed numquid quia hoc ille diues dixit, ideo quid fratres agerent uel quid paterentur illo tempore sciuit? ita illi fuit cura de uiuis, quamuis quid agerent omnino nesciret, quemadmodum est nobis cura de mortuis, quamuis quid agant utique nesciamus. nam si nihil de mortuis curaremus, non utique pro illis deo supplicaremus. denique Abraham nec Lazarum misit et Moysen ac prophetas hic eos habere respondit, quos audire deberent, ut ad illa supplicia non uenirent. ubi rursus occurrit, quomodo quid hic ageretur Abraham pater ipse nesciebat, ubi sciebat esse Moysen et prophetas, id est libros eorum, quibus homines oboediendo tormenta inferna euitarent, ubi denique nouerat diuitem illum in deliciis, pauperem uero Lazarum in laboribus doloribusque uixisse. nam et hoc illi ait: memento, fili, quia percepisti bona in uita tua, Lazarus autem mala. sciebat ergo haec, quae utique apud uiuos, non apud mortuos gesta fuerant. uerum non cum agerentur in uiuis, sed eis mortuis potuit Lazaro indicante cognoscere, ne falsum sit, quod ait propheta: Abraham nesciuit nos.