Discretion

 John Cassian lived in the 4th century and his writings, called Conferences are the greatest compendia of the monastic tradition of the desert masters.

In Book 2, one of these masters, the blessed Moses, remembers a conversation about perfection that some of the elders had with the great Antony, the original desert master.  Some thought perfection consisted chiefly in fasts and vigils.  Others, in attaining contempt for all things.  Others, in solitude.  Others, in duties of love and hospitality.  Finally, deep into the night, Antony "finally" speaks:

"All the things that you have mentioned are indeed necessary and useful for those who thirst for God and who desire to come to him.  But the innumerable falls and experiences of many people do not at all permit us to attribute the highest grace to these things.  For we often see that those who keep fasts and vigils most rigorously and who live far off in the solitude in wondrous fashion, who also deprive themselves of any belongings to such an extent that they do not so much as allow a single day's food or one denarius to be left over, and who even fulfill the demands of hospitality with the utmost devotion, are so suddenly deceived that they are unable to bring to a satisfactory conclusion the work that they have begun, and they cap off the highest fervor and a praiseworthy way of life with a disreputable end.  Therefore we would be able to know clearly what was the best way to come to God if we carefully sought out the reason for the ruin and deception of these people.  For although the works of the aforesaid virtues abounded in them, the lack of discretion by itself did not permit those works to endure to the end.  Nor can another reason be found for their fall, except that they were less well instructed by the elders and were utterly unable to grasp the meaning of discretion, which avoids excess of any kind and teaches the monk always to proceed along the royal road and does not let him be inflated by virtues on the right hand - that is, in an excess of fervor to exceed the measure of a justifiable moderation by a foolish presumption - nor let him wander off to the vices on the left hand because of a weakness for pleasure - that is, under the pretext of controlling the body, to grow soft because of a contrary lukewarmness of spirit." (Cassian, 85)

Antony depicts for us Jesus' narrow road as narrow because there is always the danger of the pendulum swing, that by avoiding one extreme too much, we end up falling into the opposite extreme, and even when we've fallen into both we can get stuck in a muddled, boring sense of moderation that is at root fearful of all future falls.  This path is narrow indeed, where it seems we would constantly be teetering toward one fall or another!  But the way of discretion remains the royal road for Antony, that for all its narrowness, royal because it is a glad road, freed from all extremes even the 'extreme' of moderation.  Antony will go on to speak about the eye of the soul.  This is key because the eye of the soul is on instruction, on wisdom, on the elders, and chiefly on Father, Son, and Holy Spirit.  Above all, not on ourselves.  To have discretion is to be wise, not because we are wise, but because we walk with the wise.