Saint John Cassian

“For it is an impossibility that the fiery motions of the body can be extinguished, before the incentives of the other chief vices are utterly rooted out: concerning which we will speak in their proper place, if God permits, separately, in different books.  But now we have to deal with Gluttony, that is the desire of the palate, against which our first battle is. He then will never be able to check the motions of a burning lust, who cannot restrain the desires of the appetite. The chastity of the inner man is shown by the perfection of this virtue. For you will never feel sure that he can strive against the opposition of a stronger enemy, whom you have seen overcome by weaker ones in a higher conflict. For of all virtues the nature is but one and the same, although they appear to be divided into many different kinds and names: just as there is but one substance of gold, although it may seem to be distributed through many different kinds of jewelry according to the skill of the goldsmith. And so he is proved to possess no virtue perfectly, who is known to have broken down in some part of them. For how can we believe that that man has extinguished the burning heats of concupiscence (which are kindled not only by bodily incitement but by vice of the mind), who could not assuage the sharp stings of anger which break out from intemperance of heart alone? Or how can we think that he has repressed the wanton desires of the flesh and spirit, who has not been able to conquer the simple fault of pride? Or how can we believe that one has trampled under foot a wantonness which is ingrained in the flesh, who has not been able to disown the love of money, which is something external and outside our own substance? In what way will he triumph in the war of flesh and spirit, who has not been man enough to cure the disease of dejection? However great a city may be protected by the height of its walls and the strength of its closed gates, yet it is laid waste by the giving up of one postern however small. For what difference does it make whether a dangerous foe makes his way into the heart of the city over high walls, and through the wide spaces of the gate, or through secret and narrow passages?”

– St. John Cassian, The Twelve Books on the Institutes of the Cœnobia, and the Remedies for the Eight Principal Faults, Book V (On the Spirit of Gluttony), Chapter XI (That bodily lusts are not extinguished except by the entire rooting out of vice).