The First Part
Chapter 21
ON THAT WHICH HAPPENS DURING PRAYER [UNTO THOSE WHO LIVE] IN SOLITUDE
Who is he that knows that delightful bending of the knees, when the tongue is silent and the heart silently utters some glorification, and its delightful emotion does not abate, 163 the body resting on the knees in silence? Blessed who eats from these things perpetually. But they do not happen at will, nor when one seeks them. This is in part the delight which is given for the consolation of him who walks without a blame before the Lord in the course of solitude.
If he continues this course in all simplicity, and seeks the purity of his service, and if his behaviour is worthy, after some time he will be deemed worthy of the things mentioned above. As to those who are novices in this course and who have a fixed aim, Grace at first will make them apt for the taste of these and similar things by recitation and it will draw their thoughts towards its self, away from earthly thoughts. Then they will work and wake and pray without becoming fatigued. Unto those who are trained to some extent in the mysteries of solitude, apperceptive power during prayer and service will be imparted.
Source: Wensinck, A. J., trans. 1923. Mystic Treatises by Isaac of Nineveh. Amsterdam: Uitgave Der Koninklijke Akademie Van Wetenschappen. Pages 110-111.