Chapter 5 - That a common rule of fasting cannot be observed by all.
When it comes to fasting, it is not easy to observe a common discipline, since our physical stregnth varies and such strength cannot be produced by sheer will-power, as in the case of other virtues. It is because it does not depend on mental effort alone but requires the cooperation of the body, that we have come to agree on the following rule which was handed down to us: the time, maner and quantity of food should be varied according to physical strength, age and sex, whereas all alike can observe a common rule of self-denial to strengthen the mind and steel the soul. Not everyone is able to prolong the fast all week, but they can put off eating for three days or perhaps two. Many, enfeebled through sickness, particularly trough old age, cannot even endure to fast one day until sundown without great discomfort. Not for all is it enough to eat steeped vegetables, nor can everyone endure to live on nothing but leeks, and it is not universally acceptable to eat dry bread. One many may find that he can eat two pounds of bread without feeling full, whereas another may be glutted with one pound or even six ounces. Nevertheless there is one goal in mind, that no one should be sated and burdened with having eaten his fill. It is not just the quality of the food but also its quantity which can blunten the point of the soul, and when the mind is fattened with the flesh it kindles a dangerously fiery spark of vice.
. . .
Chapter 7 - How bodily weakness is no hindrance to a pure soul.
The needs of the flesh are no hindrance to purity of heart, if we consume only what our bodily weakness requires, not what the appetite demands. We can observe that those who totally abstain from the richer types of food, rather than accepting them in moderation out of necessity, and deny themselves totally through longing for self-control, are more liable to fall than those who take whatever food their weakness needs, and keep to a moderate amount. Bodily frailty has its own reward of continence, as long as we restrain ourselves while we are still hungry for the nourishing food we allow ourselves, and accept as much food as on careful considerationw ould be enough to live on, without giving in to what greed demands. More appetising food which promotes physical health does nto hinder bodily purity if taken in moderation. Indeed whatever we eat for the sake of nourishment is used up in averting illness and exhaustion. Hence we need not be deprived of the virtue of restraint whatever the state of our health, as long as we do not eat as much as we feel like.
Chapter 8 - How food should be taken with the aim of perfect self-control.
The opinion of the Fathers is very true and admirable, that the standard of fasting and self-denial depends on moderation in abstinence and penance. The common goal of perfect virtue for all is that in eating the food which we need to sustain our bodies, we check ourselves while still hungry. No matter how frail someone may be physically, he can acquire as much virtue as the sturdy and healthy, if he mortifies the mental desires which his weak body is incapable of fulfilling. St. Paul says, (Rom 3:14) "do not worry about the body and its urgings." He is not totally prohibiting care for the body, but prevents us from giving in to its passions. He prohibits luxurious pampering of the flesh, but does not exclude necessary care for life. The former, lest we tumble into vile and passionate practices through pandering to the flesh; the latter lest the body, weakened by our fault, be unable to carry out its necessary and religious work.
. . .
Chapter 20 - That a monk must not eat outside the usual time, if he wishes to qualify for the interior conflict.
That is why a monk who aspires to the interior conflict must first of all take this precaution, that he be not enticed by the enjoyment of any food and drink to indulge himself in partaking alone, away from the table, before the proper procession at the time for the common meal. Nor, when the meal is over, should he allow himself to take the least morsel. In the same way he whould observe the statutory hours and period of sleep. By this practice the lusts of the mind are restrained, and sexual vice eliminated. For how could someone who is unable to restrain his appetite for extra food be capable of quenching the burnings lusts of the flesh? And how could someone who cannot control desires that are public and petty, be sufficiently self-disciplined to defeat secret desires that no one would witness? Therefore strength of character can be tested on particular occasions and by any sort of temptation, for if he be conquered by trivialities in public, let his own conscience judge whether he will endure overwhelming temptation in private.
St. John Cassian, The Monastic Institues, Book V