Prayer is of three sorts

From a Letter of St. Catherine of Siena to her niece Eugenia, a nun at Montepulciano

Prayer is of three sorts. The one is perpetual: it is the holy perpetual desire, which prays in the sight of God, whatever thou art doing; for this desire directs all thy works, spiritual and corporal, to His honour, and therefore it is called perpetual. Of this it seems that Saint Paul the glorious was talking when he said: Pray without ceasing. The other kind is vocal prayer, when the offices or other prayers are said aloud. This is ordained to reach the third--that is, mental prayer: your soul reaches this when it uses vocal prayer in prudence and humility, so that while the tongue speaks the heart is not far from God. But one must exert one's self to hold and establish one's heart in the force of divine charity. And whenever one felt one's mind to be visited by God, so that it was drawn to think of its Creator in any wise, it ought to abandon vocal prayer, and to fix its mind with the force of love upon that wherein it sees God visit it; then, if it has time, when this has ceased, it ought to take up the vocal prayer again, in order that the mind may always stay full and not empty. And although many conflicts of diverse kinds should abound in prayer, and darkness of mind with much confusion, the devil making the soul feel that her prayer was not pleasing to God--nevertheless, she ought not to give up on account of those conflicts and shadows, but to abide firm in fortitude and long perseverance, considering that the devil so does to draw her away from prayer the mother, and God permits it to test the fortitude and constancy of that soul. Also, in order that by those conflicts and shadows she may know herself not to be, and in the goodwill which she feels preserved within her may know the goodness of God, Who is Giver and Preserver of good and holy wills: such wills as are not vouchsafed to all who want them. By this means she attains to the third and last--mental prayer, in which she receives the reward for the labours she underwent in her imperfect vocal prayer. Then she tastes the milk of faithful prayer. She rises above herself--that is, above the gross impulses of the senses--and with angelic mind unites herself with God by force of love, and sees and knows with the light of thought, and clothes herself with truth. She is made the sister of angels; she abides with her Bridegroom on the table of crucified desire, rejoicing to seek the honour of God and the salvation of souls; since well she sees that for this the Eternal Bridegroom ran to the shameful death of the Cross, and thus fulfilled obedience to the Father, and our salvation. This prayer is surely a mother, who conceives virtues by the love of God, and brings them forth in the love of the neighbour. Where dost thou show love, faith, and hope, and humility? In prayer. For thou wouldst never take pains to seek the thing which thou didst not love; but he who loves would ever be one with what he loves--that is, God. By means of prayer thou askest of Him thy necessity; for knowing thyself--the knowledge on which true prayer is founded--thou seest thyself to have great need. Thou feelest thyself surrounded by thine enemies--by the world with its insults and its recalling of vain pleasures, by the devil with his many temptations, by the flesh with its great rebellion and struggle against the spirit. And thou seest that in thyself thou art not; not being, thou canst not help thyself; and therefore thou dost hasten in faith to Him who is, who can and will help thee in thine every need, and thou dost hopefully ask and await His aid. Thus ought prayer to be made, if thou wishest to have that which thou awaitest. Never shall any just thing be denied thee which thou askest in this wise from the Divine Goodness; but if thou dost in other wise, little fruit shalt thou receive. Where shalt thou feel grief in thy conscience? In prayer. Where shalt thou divest thee of the self-love which makes thee impatient in the time of insults and of other pains, and shalt clothe thee in the divine love which shall make thee patient, and shalt glory in the Cross of Christ crucified? In prayer. Where shalt thou breathe the perfume of virginity and the hunger for martyrdom, holding thee ready to give thy life for the honour of God and the salvation of souls? In this sweet mother, prayer. This will make thee an observer of thy Rule: it will seal in thy heart and mind three solemn vows which thou didst make at thy profession, leaving there the imprint of the desire to observe them until death. This releases thee from conversation with fellow-creatures, and gives thee converse with thy Creator; it fills the vessel of thy heart with the Blood of the Humble Lamb, and crowns it with flame, because with flame of love that Blood was shed. The soul receives and tastes this mother Prayer more or less perfectly, according as it nourishes itself with the food of angels--that is, with holy and true desire for God, raising itself on high, as I said, to receive it upon the table of the most sweet Cross. Therefore I said to thee that I desired to see thee nourished with angelic food, because I see not that in otherwise thou couldst be a true bride of Christ crucified, consecrated to Him in holy religion. So do that I may see thee a jewel precious in the sight of God. And do not go about wasting thy time. Bathe and drown thee in the sweet Blood of thy Bridegroom. I say no more. Remain in the holy and sweet grace of God. Sweet Jesus, Jesus Love.