Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XXI. The answer concerning the value and measure of well-proved abstinence.

Chapter XXI.

The answer concerning the value and measure of well-proved abstinence.

Moses: If you want to test the force of this rule, keep to this limit continually, never departing from it by taking any cooked food even on Sunday or Saturday, or on the occasions of the arrival of any of the brethren; for the flesh, refreshed by these exceptions, is able not only to support itself through the rest of the week on a smaller quantity, but can also postpone all refreshment without difficulty, as it is sustained by the addition of that food which it has taken beyond the limit; while the man who has always been satisfied with the full amount of the above-mentioned measure will never be able to do this, nor to put off breaking his fast till the morrow. For I remember that our Elders (and I recollect that we ourselves also often had the same experience) found it so hard and difficult to practise this abstinence, and observed the rule laid down with such pain and hunger that it was almost against their will and with tears and lamentation that they set this limit to their meals.