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]