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.

Catherine of Siena (1347-1380): Letter to Sister Eugenia.