Objection 1. It would seem that the saints have no knowledge of our prayers. For a gloss on Isaiah 62:16, "Thou art our father and Abraham hath not known us, and Israel hath been ignorant of us," says that "the dead saints know not what the living, even their own children, are doing." This is taken from Augustine (De Cura pro Mort. xiii), where he quotes the aforesaid authority, and the following are his words: "If such great men as the patriarchs knew not what was happening to the people begotten of them, how can the dead occupy themselves in watching and helping the affairs and actions of the living?" Therefore the saints cannot be cognizant of our prayers.
Objection 2. Further, the following words are addressed to King Joas (2 Kings 22:20): "Therefore" (i.e. because thou hast wept before Me), "I will gather thee to thy fathers... that thy eyes may not see all the evils which I will bring upon this place." But Joas would have gained no such advantage from his death if he were to know after death what was happening to his people. Therefore the saints after death know not our actions, and thus they are not cognizant of our prayers.
Objection 3. Further, the more perfect a man is in charity, the more he succors his neighbor when the latter is in danger. Now the saints, in this life, watch over their neighbor, especially their kinsfolk, when these are in danger, and manifestly assist them. Since then, after death, their charity is much greater, if they were cognizant of our deeds, much more would they watch over their friends and kindred and assist them in their needs: and yet, seemingly, they do not. Therefore it would seem that our deeds and prayers are not known to them.
Objection 4. Further, even as the saints after death see the Word, so do the angels of whom it is stated (Matthew 18:10) that "their angels in heaven always see the face of My Father." Yet the angels through seeing the Word do not therefore know all things, since the lower angels are cleansed from their lack of knowledge by the higher angels [Cf. I, 106, 1], as Dionysius declares (Coel. Hier. vii). Therefore although the saints see the Word, they do not see therein our prayers and other things that happen in our regard.
Objection 5. Further, God alone is the searcher of hearts. Now prayer is seated chiefly in the heart. Therefore it belongs to God alone to know our prayers. Therefore our prayers are unknown to the saints.
On the contrary, Gregory, commenting on Job 14:21, "Whether his children come to honor or dishonor, he shall not understand," says (Moral. xii): "This does not apply to the souls of the saints, for since they have an insight of Almighty God's glory we must nowise believe that anything outside that glory is unknown to them." Therefore they are cognizant of our prayers. Further, Gregory says (Dial. ii): "All creatures are little to the soul that sees God: because however little it sees of the Creator's light, every created thing appears foreshortened to it." Now apparently the chief obstacle to the souls of the saints being cognizant of our prayers and other happenings in our regard is that they are far removed from us. Since then distance does not prevent these things, as appears from the authority quoted, it would seem that the souls of the saints are cognizant of our prayers and of what happens here below.
Further, unless they were aware of what happens in our regard they would not pray for us, since they would be ignorant of our needs. But this is the error of Vigilantius, as Jerome asserts in his letter against him. Therefore the saints are cognizant of what happens in our regard.
I answer that, The Divine essence is a sufficient medium for knowing all things, and this is evident from the fact that God, by seeing His essence, sees all things. But it does not follow that whoever sees God's essence knows all things, but only those who comprehend the essence of God [Cf. I, 12, 7,8]: even as the knowledge of a principle does not involve the knowledge of all that follows from that principle unless the whole virtue of the principle be comprehended. Wherefore, since the souls of the saints do not comprehend the Divine essence, it does not follow that they know all that can be known by the Divine essence--for which reason the lower angels are taught concerning certain matters by the higher angels, though they all see the essence of God; but each of the blessed must needs see in the Divine essence as many other things as the perfection of his happiness requires. For the perfection of a man's happiness requires him to have whatever he will, and to will nothing amiss: and each one wills with a right will, to know what concerns himself. Hence since no rectitude is lacking to the saints, they wish to know what concerns themselves, and consequently it follows that they know it in the Word. Now it pertains to their glory that they assist the needy for their salvation: for thus they become God's co-operators, "than which nothing is more Godlike," as Dionysius declares (Coel. Hier. iii). Wherefore it is evident that the saints are cognizant of such things as are required for this purpose; and so it is manifest that they know in the Word the vows, devotions, and prayers of those who have recourse to their assistance.
Reply to Objection 1. The saying of Augustine is to be understood as referring to the natural knowledge of separated souls, which knowledge is devoid of obscurity in holy men. But he is not speaking of their knowledge in the Word, for it is clear that when Isaias said this, Abraham had no such knowledge, since no one had come to the vision of God before Christ's passion.
Reply to Objection 2. Although the saints, after this life, know what happens here below, we must not believe that they grieve through knowing the woes of those whom they loved in this world: for they are so filled with heavenly joy, that sorrow finds no place in them. Wherefore if after death they know the woes of their friends, their grief is forestalled by their removal from this world before their woes occur. Perhaps, however, the non-glorified souls would grieve somewhat, if they were aware of the distress of their dear ones: and since the soul of Josias was not glorified as soon as it went out from his body, it is in this respect that Augustine uses this argument to show that the souls of the dead have no knowledge of the deeds of the living.
Reply to Objection 3. The souls of the saints have their will fully conformed to the Divine will even as regards the things willed. and consequently, although they retain the love of charity towards their neighbor, they do not succor him otherwise than they see to be in conformity with the disposition of Divine justice. Nevertheless, it is to be believed that they help their neighbor very much by interceding for him to God.
Reply to Objection 4. Although it does not follow that those who see the Word see all things in the Word, they see those things that pertain to the perfection of their happiness, as stated above.
Reply to Objection 5. God alone of Himself knows the thoughts of the heart: yet others know them, in so far as these are revealed to them, either by their vision of the Word or by any other means.
Source: Summa Theologica (New Advent)