Support with your word the weak and the distressed in spirit whenever you can; then the hand that bears the universe will support you.
Participate with those who are suffering in heart, in passionate prayer and mourning of the heart; then before your demand a fountain of grace will be opened.
Be strenuous in prayer at all time before God, with a heart full of chaste deliberations mingled with passion; then He will preserve your mind from impure thoughts, so that the way of God be not disordered in you.
Occupy your gaze with constant intercourse with intelligent recitation of the scriptures, lest, on account of idleness, the sight of foreign things defile your look.
Do not tempt your mind, for the sake of examination, by consideration of impure seductive thoughts, thinking that you shall not he vanquished; even wise men have been perturbed in this place and deviated.
Do not take fire in your bosom….
Without severe bodily trouble, it is hard for the untrained youth to be bound under the yoke of saintliness.
The sign of the beginning of darkness of mind manifests itself in the soul by dejection, in the first place with regard to service and prayer. For it is not possible that the way in your soul towards error should be opened if you had not fallen in this point first.
Then, being bereft of God’s help — which otherwise affords a way unto Him — you will easily fall into the hands of the foes. And further, being without care for the matters of excellence, you will be carried towards the contrary things in every manner. Departing, from any side, is the beginning of approaching to the opposite one.
Let the service of excellence be firm in your soul; meditate on it and so on. Show your weakness before God at all times, lest strangers come to examine your strength while you are separated from your helper.
The service of the cross is a double one. And this is in accordance with its twofold nature which is divided into two parts: patience in face of bodily troubles, which is accomplished through the instrumentality of the anger of the soul; this is called practice; and the subtle intellectual service, in intercourse with God, constant prayer and so on, which is performed with the desiring part and called theory.
The one purifies the affectable part by the strength of zeal; the other clears the intellectual part by the influence of the love of the soul, which is the natural appetite.
Isaac the Syrian (c. 630-c. 700): Six Treatises on the Behaviour of Excellence, 1, 2, in Mystical Treatises of Isaac of Nineveh, trans. A.J. Wensinck, pp. 9-10 (slightly modified).