Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XVII. Of the reason for fasting and its value.

Chapter XVII.

Of the reason for fasting and its value.

So then let us constantly remember this description of the character of fasting, and always aim at it with all the powers of the soul, in such a way as to recognize that then only is it suitable for us if in it we preserve regard for time, its character and degree, and this not so as to set the end of our hope upon it, but so that by it we may succeed in attaining to purity of heart and Apostolical love. Therefore from this it is clear that fasting, for which not only are there special seasons appointed at which it should be practised or relaxed, but conditions and rules also laid down, is not good in its own nature, but something indifferent. But those things which are either enjoined as good by the authority of a precept, or are forbidden as bad, are never subject to any exceptions of time in such a way that sometimes we should do what is forbidden or omit what is commanded. For there is no limit set to justice, patience, soberness, modesty, love, nor on the other hand is a licence ever granted for injustice, impatience, wrath, immodesty, envy, and pride.