Objection 1. It would seem that we ought to pray to God alone. Prayer is an act of religion, as stated above (Article 3). But God alone is to be worshiped by religion. Therefore we should pray to God alone.
Objection 2. Further, it is useless to pray to one who is ignorant of the prayer. But it belongs to God alone to know one's prayer, both because frequently prayer is uttered by an interior act which God alone knows, rather than by words, according to the saying of the Apostle (1 Corinthians 14:15), "I will pray with the spirit, I will pray also with the understanding": and again because, as Augustine says (De Cura pro mortuis xiii) the "dead, even the saints, know not what the living, even their own children, are doing." Therefore we ought to pray to God alone.
Objection 3. Further, if we pray to any of the saints, this is only because they are united to God. Now some yet living in this world, or even some who are in Purgatory, are closely united to God by grace, and yet we do not pray to them. Therefore neither should we pray to the saints who are in Paradise.
On the contrary, It is written (Job 5:1), "Call... if there be any that will answer thee, and turn to some of the saints."
I answer that, Prayer is offered to a person in two ways: first, as to be fulfilled by him, secondly, as to be obtained through him. On the first way we offer prayer to God alone, since all our prayers ought to be directed to the acquisition of grace and glory, which God alone gives, according to Psalm 83:12, "The Lord will give grace and glory." But in the second way we pray to the saints, whether angels or men, not that God may through them know our petitions, but that our prayers may be effective through their prayers and merits. Hence it is written (Apocalypse 8:4) that "the smoke of the incense," namely "the prayers of the saints ascended up before God." This is also clear from the very style employed by the Church in praying: since we beseech the Blessed Trinity "to have mercy on us," while we ask any of the saints "to pray for us."
Reply to Objection 1. To Him alone do we offer religious worship when praying, from Whom we seek to obtain what we pray for, because by so doing we confess that He is the Author of our goods: but not to those whom we call upon as our advocates in God's presence.
Reply to Objection 2. The dead, if we consider their natural condition, do not know what takes place in this world, especially the interior movements of the heart. Nevertheless, according to Gregory (Moral. xii, 21), whatever it is fitting the blessed should know about what happens to us, even as regards the interior movements of the heart, is made known to them in the Word: and it is most becoming to their exalted position that they should know the petitions we make to them by word or thought; and consequently the petitions which we raise to them are known to them through Divine manifestation.
Reply to Objection 3. Those who are in this world or in Purgatory, do not yet enjoy the vision of the Word, so as to be able to know what we think or say. Wherefore we do not seek their assistance by praying to them, but ask it of the living by speaking to them.
Source: Summa Theologica (New Advent)