Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter VIII. How those who are perfect ought not to make any promises absolutely, and whether decisions can be reversed without sin.

Chapter VIII.

How those who are perfect ought not to make any promises absolutely, and whether decisions can be reversed without sin.

Joseph: It is good indeed and right and altogether in accordance with our profession, for us effectually to perform what we decided to do in the case of any promise. Wherefore a monk ought not to make any promise hastily, lest he may be forced to do what he incautiously promised, or if he is kept back by consideration of a sounder view, appear as a breaker of his promise. But because at the present moment our purpose is to treat not so much of a state of health as of the cure of sickness we must with salutary counsel consider not what you ought to have done in the first instance, but how you can escape from the rocks of this perilous shipwreck. When then no chains impede us and no conditions restrict us, in the case of a comparison of good things, if a choice is proposed, that which is most advantageous should be preferred: but when some detriment and loss stands in the way, in a comparison of things to our hurt, that should be sought which exposes us to the smallest loss. Further, as your assertion shows, when your heedless promise has brought you to this state that in either case some serious loss and inconvenience must result to you, the will in choosing should incline to that side which involves a loss that is more tolerable, or can be more easily made up for by the remedy of making amends. If then you think that you will get more good for your spirit by staying here than what accrued to you from your life in that monastery, and that the terms of your promise cannot be fulfilled without the loss of great good, it is better for you to undergo the loss from a falsehood and an unfulfilled promise (as it is done once for all, and need not any longer be repeated or be the cause of other sins) than for you to incur that loss, through which you say that your state of life would become colder, and which would affect you with a daily and unceasing injury. For a careless promise is changed in such a way that it may be pardoned or indeed praised, if it is turned into a better path, nor need we take it as a failure in consistency, but as a correction of rashness, whenever a promise that was faulty is corrected. And all this may be proved by most certain witness from Scripture, that for many the fulfilment of their promise has led to death, and on the other hand that for many it has been good and profitable to have refused it.