John Cassian, Conferences XVIII–XXIV XXI.13: What kind of good fasting is

Wherefore we must now inquire what we ought to hold about the state of fasting, whether we meant that it was good in the same sort of way as justice, prudence, fortitude and temperance, which cannot possibly be made anything else, or whether it is something indifferent which sometimes is useful when done, and may be sometimes omitted without condemnation; and which sometimes it is wrong to do, and sometimes laudable to omit. For if we hold fasting to be included in that list of virtues, so that abstinence from food is placed among those things which are good in themselves, then certainly the partaking of food will be bad and wrong. For whatever is the opposite of that which is in its own nature good, must certainly be held to be in its own nature bad. But this the authority of Holy Scripture does not allow to us to lay down. For if we fast with such thoughts and intentions, so as to think that we fall into sin by taking food, we shall not only gain no advantage by our abstinence but shall actually contract grievous guilt and fall into the sin of impiety, as the Apostle says: “Abstaining from meats which God has created to be received with thanksgiving by the faithful and those who know the truth. For every creature of God is good, and nothing to be refused if it is partaken of with thanksgiving.” For “if a man thinks that a thing is common, to him it is common.” And therefore we never read that anyone is condemned simply for taking food, but only when something was joined with it or followed afterwards, for which he deserved condemnation.