Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter VIII. Of the different characters of prayer.

Chapter VIII.

Of the different characters of prayer.

Isaac: I imagine that all kinds of prayers cannot be grasped without great purity of heart and soul and the illumination of the Holy Spirit. For there are as many of them as there can be conditions and characters produced in one soul or rather in all souls. And so although we know that owing to our dulness of heart we cannot see all kinds of prayers, yet we will try to relate them in some order, as far as our slender experience enables us to succeed. For according to the degree of the purity to which each soul attains, and the character of the state in which it is sunk owing to what happens to it, or is by its own efforts renewing itself, its very prayers will each moment be altered: and therefore it is quite clear that no one can always offer up uniform prayers. For every one prays in one way when he is brisk, in another when he is oppressed with a weight of sadness or despair, in another when he is invigorated by spiritual achievements, in another when cast down by the burden of attacks, in another when he is asking pardon for his sins, in another when he asks to obtain grace or some virtue or else prays for the destruction of some sin, in another when he is pricked to the heart by the thought of hell and the fear of future judgment, in another when he is aglow with the hope and desire of good things to come, in another when he is taken up with affairs and dangers, in another when he is in peace and security, in another when he is enlightened by the revelation of heavenly mysteries, and in another when he is depressed by a sense of barrenness in virtues and dryness in feeling.