The disciple: What is the acme of all labours of asceticism, which a man when he has reached it, recognises as the summit of his course?
The teacher: When he is deemed worthy of constant prayer. When he has reached this, he has touched the end of all virtues and forthwith he has a spiritual dwelling place. If a man has not received in truth the gift of the Comforter, it is not possible for him to accomplish constant prayer in quiet. When the spirit takes its dwelling place in a man he does not cease to pray because the spirit will constantly pray in him. Then, neither when he sleeps, nor when he is awake, will prayer be cut off from his soul; but when he eats and when he drinks, when he lies down or when he does any work, even when he is immersed in sleep, the perfumes of prayer will breathe in his soul spontaneously. And henceforth, he will not possess prayer at limited times but always; and when he has outward rest, even then prayer is ministered unto him secretly. For the silence of the serene is prayer, says a man clad with Christ. For their deliberations are divine impulses. The motions of the pure mind are quiet voices with which they secretly chant psalms to the Invisible One.
[Mar Isaac the Syrian, Ascetical Homilies, XXXV]
Prayer is a beseeching for, a caring for, a longing for something, either a liberation from the evil things here or [in the world] to come, or a desire for promised things, or a demand for something by which man wishes to be brought nearer to God. In these emotions are included all habits of prayer. But its being pure or not depends upon the following circumstances.
If, when the spirit is prepared to offer one of the emotions we have enumerated, any foreign deliberation or distraction mingles itself with it, prayer is called non-pure, because it has brought upon the altar of the Lord an animal which it is not allowed [to offer], the altar which is an upright, intelligible heart.
But when the spirit gives itself with longing to one of these emotions, in accordance to the necessity of the case, at the time of beseeching, and when on account of its alacrity the gaze of the emotion is directed by the eye of faith beyond the curtain of the heart, the entrances of the soul are closed thereby against the foreign deliberations which are called strangers, whom the law does not allow to enter the tabernacle. This is called the accepted offering of the heart and pure prayer. Its boundaries are to this point. What lies beyond cannot be called prayer.
[Mar Isaac the Syrian, Ascetical Homilies, XXII]
Whenever you find your course is in peace, without variations, be suspicious. For you are deviating from the divine ways trodden by the weary footsteps of the saints. The more you proceed on the way towards the city of the Kingdom and approach it’s neighbourhoods, this will be the sign: that you meet hard temptations. And the more you approach, the more you will find difficulties.
So, whenever the soul on its way perceives varying states which cause difficulty you must know that your soul has secretly been advanced to a higher state and that it has acquired a gift of increase in comparison with the degree it occupied before.
The hard temptation into which God brings the soul are in accordance with the greatness of His gifts. If there is a weak soul which is not able to bear a very hard temptation and God deals meekly with it, then know with certainty that, as it is not capable of bearing a hard temptation, so it is not worthy of a large gift. As great temptations have been withdrawn from it, so large gifts are also withdrawn from it. God never gives a large gift and small temptations. So temptations are to be classed in accordance with gifts. Thus from the hardships to which you have been subjected you may understand the measure of the greatness which your soul has reached. In accordance with affection is consolation.
[Mar Isaac the Syrian, Ascetical Homilies]