The Catechism of the Summa - XXIX. OF RELIGION AND ITS INTERIOR ACTS: OF DEVOTION; OF PRAYER, ITS NATURE AND NECESSITY; OF THE "OUR FATHER" OR THE LORD'S PRAYER, AND ITS EFFICACIOUSNESS (B)

(B)  
What is prayer?

Prayer, understood in its widest sense and in so far as it is addressed to God, is an act of the practical reason by which, under the form of supplication, we desire to lead God to grant what we ask (LXXXIII. 1).
 
But is this a reasonable thing?

Yes, of a truth it is; for there is nothing more reasonable and more in harmony with our nature than to do this (LXXXIII. 2).
 
How can this be shown?

By the following considerations: since we are by nature rational beings, we have need of considering in the greatest degree what God is and what we are. But we are filled with miseries; and He is the source of all good. The more intimately we know then our own misery in all its details, and that God only is capable of succouring our needs, the more we shall come to know what we ought to be, that is to know what our very nature has need of; and this is precisely what prayer effects. It is, moreover, the more perfect when it makes us the more conscious of our misery and of the goodness of God, which is the remedy of that misery. It is for this reason that God in His mercy wishes us to pray, and He has even determined that certain boons shall not be conferred upon us unless we ask Him for them (LXXXIII. 2).
 
It is then God's will that we are fulfilling when we endeavour by prayer to lead Him to grant what we ask?

Yes, it is God's will that we should do this; that is of course whenever what we ask of Him is for our own true good.

Next - The Catechism of the Summa - XXIX. OF RELIGION AND ITS INTERIOR ACTS: OF DEVOTION; OF PRAYER, ITS NATURE AND NECESSITY; OF THE "OUR FATHER" OR THE LORD'S PRAYER, AND ITS EFFICACIOUSNESS (C)