The answer how we can gain stability of heart or of thoughts.
Isaac: Although, in our former discussion on the character of prayer, enough was, as I think, said on this subject, yet as you want it repeated to you again, I will give you a brief instruction on steadfastness of heart. There are three things which make a shifting heart steadfast, watchings, meditation, and prayer, diligence in which and constant attention will produce steadfast firmness of mind. But this cannot be secured in any other way unless all cares and anxieties of this present life have been first got rid of by indefatigable persistence in work dedicated not to covetousness but to the sacred uses of the monastery, that we may thus be able to fulfil the Apostles command: “Pray without ceasing.” 1681 For he prays too little, who is accustomed only to pray at the times when he bends his knees. But he never prays, who even while on his bended knees is distracted by all kinds of wanderings of heart. And therefore what we would be found when at our prayers, that we ought to be before the time of prayer. For at the time of its prayers the mind cannot help being affected by its previous condition, and while it is praying, will be either transported to things heavenly, or dragged down to earthly things by those thoughts in which it had been lingering before prayer.
Thus far did Abbot Isaac carry on his Second Conference on the character of Prayer to us astonished hearers; whose instruction on the consideration of that verse quoted above (which he gave as a sort of outline for beginners to hold) we greatly admired, and wished to follow very closely, as we fancied that it would be a short and easy method; but we have found it even harder to observe than that system of ours by which we used formerly to wander here and there in varied meditations through the whole body of the Scriptures without being tied by any chains of perseverance. It is then certain that no one is kept away from perfection of heart by not being able to read, nor is rustic simplicity any hindrance to the possession of purity of heart and mind, which lies close at hand for all, if only they will by constant meditation on this verse keep the thoughts of the mind safe and sound towards God.
1 Thess. v. 17.