One should perform the Prayer of Jesus aloud but not loudly, sufficient so that one can hear it. One should not at the time of prayer incline one's thought here and there to worldly , corruptible
things, but should remain without laziness in the memory of this Prayer alone.
The Prayer is nothing else but a division between the visible and invisible worlds. Wherefore, one must enclose one's mind in the Prayer. Where
the body stands, there also the mind should be with it, without having any thought at the time of prayer. The Holy Fathers say, "If anyone prays with his lips but is careless about his mind, he is
laboring in vain, for God heeds the mind and not much speaking.
Mental prayer does not allow that there be in
the mind any fantasy or unclean thought. If one does not become accustomed to the mental Jesus Prayer, he cannot have ceaseless prayer. If the
Jesus Prayer becomes one's habit and enters his heart, it will then flow as water from a spring. At all times, everywhere and no matter what one
does, it will always urge him on, be he awake or asleep. When his body begins to sleep or drowse, even then it will awaken him, gushing out of his heart and never ceasing.
Therefore this Prayer is great, never left off, so that when it is being said, although the lips may grow faint, and the body may drowse, the spirit never sleeps. When some essential work is performed diligently or thoughts powerfully attack the mind or sleep overcomes one, then one must pray fervently with the lips and tongue, that the mind might heed the voice. And when the mind is in peace and calm from thoughts, one may pray with it alone.
This path of prayer is the swifter one to salvation than by means of psalms, canons, and the usual prayers....
St. Paisius Velichkovsky