Objection 1. It would seem that separated souls know what takes place on earth; for otherwise they would have no care for it, as they have, according to what Dives said (Luke 16:27-28), "I have five brethren... he may testify unto them, lest they also come into the place of torments." Therefore separated souls know what passes on earth.
Objection 2. Further, the dead often appear to the living, asleep or awake, and tell them of what takes place there; as Samuel appeared to Saul (1 Samuel 28:11). But this could not be unless they knew what takes place here. Therefore they know what takes place on earth.
Objection 3. Further, separated souls know what happens among themselves. If, therefore, they do not know what takes place among us, it must be by reason of local distance; which has been shown to be false (7).
On the contrary, It is written (Job 14:21): "He will not understand whether his children come to honor or dishonor."
I answer that, By natural knowledge, of which we are treating now, the souls of the dead do not know what passes on earth. This follows from what has been laid down (4), since the separated soul has knowledge of singulars, by being in a way determined to them, either by some vestige of previous knowledge or affection, or by the Divine order. Now the souls departed are in a state of separation from the living, both by Divine order and by their mode of existence, whilst they are joined to the world of incorporeal spiritual substances; and hence they are ignorant of what goes on among us. Whereof Gregory gives the reason thus: "The dead do not know how the living act, for the life of the spirit is far from the life of the flesh; and so, as corporeal things differ from incorporeal in genus, so they are distinct in knowledge" (Moral. xii). Augustine seems to say the same (De Cura pro Mort. xiii), when he asserts that, "the souls of the dead have no concern in the affairs of the living."
Gregory and Augustine, however, seem to be divided in opinion as regards the souls of the blessed in heaven, for Gregory continues the passage above quoted: "The case of the holy souls is different, for since they see the light of Almighty God, we cannot believe that external things are unknown to them." But Augustine (De Cura pro Mort. xiii) expressly says: "The dead, even the saints do not know what is done by the living or by their own children," as a gloss quotes on the text, "Abraham hath not known us" (Isaiah 63:16). He confirms this opinion by saying that he was not visited, nor consoled in sorrow by his mother, as when she was alive; and he could not think it possible that she was less kind when in a happier state; and again by the fact that the Lord promised to king Josias that he should die, lest he should see his people's afflictions (2 Kings 22:20). Yet Augustine says this in doubt; and premises, "Let every one take, as he pleases, what I say." Gregory, on the other hand, is positive, since he says, "We cannot believe." His opinion, indeed, seems to be the more probable one--that the souls of the blessed who see God do know all that passes here. For they are equal to the angels, of whom Augustine says that they know what happens among those living on earth. But as the souls of the blessed are most perfectly united to Divine justice, they do not suffer from sorrow, nor do they interfere in mundane affairs, except in accordance with Divine justice.
Reply to Objection 1. The souls of the departed may care for the living, even if ignorant of their state; just as we care for the dead by pouring forth prayer on their behalf, though we are ignorant of their state. Moreover, the affairs of the living can be made known to them not immediately, but the souls who pass hence thither, or by angels and demons, or even by "the revelation of the Holy Ghost," as Augustine says in the same book.
Reply to Objection 2. That the dead appear to the living in any way whatever is either by the special dispensation of God; in order that the souls of the dead may interfere in affairs of the living--and this is to be accounted as miraculous. Or else such apparitions occur through the instrumentality of bad or good angels, without the knowledge of the departed; as may likewise happen when the living appear, without their own knowledge, to others living, as Augustine says in the same book. And so it may be said of Samuel that he appeared through Divine revelation; according to Sirach 46:23, "he slept, and told the king the end of his life." Or, again, this apparition was procured by the demons; unless, indeed, the authority of Ecclesiasticus be set aside through not being received by the Jews as canonical Scripture.
Reply to Objection 3. This kind of ignorance does not proceed from the obstacle of local distance, but from the cause mentioned above.
Source: Summa Theologica (New Advent)