Even as the whole force of the laws and the commandments given by God to men terminate in the purity of the heart, according to the word of the Fathers, so all the modes and forms of prayer which men pray to God terminate in pure prayer. For sighs, prostrations, heart-felt supplications, sweet cries of lamentation, and all the other forms of prayer have, as I have said, their boundary and the extent of their domain in pure prayer. But once the mind crosses this boundary, from the purity of prayer even to that which is within, it no longer possesses prayer, or movement, or weeping, or dominion, or free will, or supplication, or desire, or fervent longing for things hoped for in this life or in the age to come. Therefore, there exists no prayer beyond pure prayer. Every movement and every form of prayer lead the mind this far by the authority of the free will; for this reason there is a struggle in prayer. But beyond this boundary there is awestruck wonder and not prayer.
The Ascetical Homilies of Saint Isaac the Syrian (I, 23), translated by the Holy Transfiguration Monastery, Boston, 1984. 116.