What is the link between discernment and moderation?

Abba Moses says, “We ought then with all our might to strive for the virtue of discretion by the power of humility, as it will keep us uninjured by either extreme, for there is an old saying ἀκρότητες ἰσότητες [ akrotētes isotētes ], i.e., extremes meet. For excess of fasting and gluttony come to the same thing, and an unlimited continuance of vigils is equally injurious to a monk as the torpor of a deep sleep: for when a man is weakened by excessive abstinence he is sure to return to that condition in which a man is kept through carelessness and negligence, so that we have often seen those who could not be deceived by gluttony, destroyed by excessive fasting … wherefore as the apostle says ‘with the arms of righteousness on the right hand and on the left’ (2 Cor. 6:7), we pass on with due moderation, and walk between the two extremes, under the guidance of discretion … For I remember that I had so often resisted the desire for food, that having abstained from taking any for two or three days, my mind was not troubled even by the recollection of any eatables and also that sleep was by the assaults of the devil so far removed from my eyes, that for several days and nights I used to pray the Lord to grant a little sleep to my eyes; and then I felt that I was in greater peril from the want of food and sleep than from struggling against sloth and gluttony. And so as we ought to be careful not to fall into dangerous effeminacy through desire for bodily gratification, nor indulge ourselves with eating before the right time nor take too much, so also we ought to refresh ourselves with food and sleep at the proper time even if we dislike it. For the struggle in each case is caused by the devices of the enemy; and excessive abstinence is still more injurious to us than careless satiety: for from this latter the intervention of a healthy compunction will raise us to the right measure of strictness, and not from the former … for as the mind which is enfeebled for lack of food loses vigour in praying, while it is worn out with excessive weakness of the flesh and forced to doze, so again when weighed down with over-eating it cannot pour forth to God pure and free prayers: nor will it succeed in preserving uninterruptedly the purity of its chastity, while even on those days on which it seems to chastise the flesh with severer abstinence, it feeds the fire of carnal desire with the fuel of the food that it has already taken.”⁷⁷¹