Prudence in spiritual matters recognizes the need for flexibility instead of expecting everyone to follow a particular, and very rigid, discipline. Not everyone, in all places, and at all times, should be doing the exact same thing. Indeed, it can be counter-productive, if not spiritually dangerous, to try to create universal, highly-generalized disciplines. This is because disciplines should be established to deal with the needs of the person or people involved, and those needs are not always one and the same with everyone. Often, those who like to enforce a particular discipline do so, not because they have any wisdom or spiritual knowledge, but because they have power and like to enforce their desires upon others. While some, perhaps, might be helped by them, and if they can be found, used to suggest there is more value to such a discipline than is actually there, what is important, and often neglected, is all those who find such forced discipline hindering or reversing the spiritual progress of those involved. This is why trying to over-regulate Christian lives with more and more disciplines, obligating people to disciplines which have no connection to their lives, can do more harm than good, just as it would do more harm than good making sure everyone went to their dentist and had fillings and crowns put in every tooth, because most people will not need to do so. Since we can see how it would be malpractice for a dentist to follow such an expectation, we should likewise be able to see it can become spiritual malpractice or abuse when various disciplines are expected to be followed by everyone. Those who suffer such abuse enough will either run away from it, once they recognize it, and flourish elsewhere, or they will try to follow what they are told to do, and will suffer greatly, internally if not externally, for doing so. This is why prudence is necessary, for it works to counteract such an overbearing form of spiritual abuse. When such prudence is not only ignored, but ridiculed, we find abuse, not spiritual well-being, flourishes within the institution or community bound by outrageous disciplines. Eventually, such abuse will become so apparent by everyone, the system which established it, including the institutions which support them, will crumble in upon themselves. This is why, as reported by St John Cassian, the desert fathers and mothers knew it was best not to try to create such universal discipline but to find ways to deal with the needs of everyone on their own footing, in their own context:
It has therefore been determined that none of these things is a permanent good, except when it is carried out at the right time and in the correct fashion. Thus, the very things that turn out well now, since they were done at the right time, are found to be disadvantageous and harmful if they are tried at an inopportune or inappropriate moment. The only exception is to those things that are essentially and of themselves either good or bad and that can never be turned to their contraries, such as justice, prudence, fortitude, temperance, and the other virtues, and, on the other hand, the vices which can never be understood differently. But if they can sometimes have different effects, so that they are found to be good or bad in accordance with the character of those who are exercising them, they are perceived not in absolute terms relative to their nature but as sometimes advantageous and sometimes harmful in keeping with the disposition of the one exercising them and with the opportuneness of the moment. 
Fasting can be very useful for some people. Late night vigils can be good for others. But both can be dangerous and harmful, especially when promoted to an extreme. Discipline should be developed either to help someone deal with root problems in their lives, or to develop various gifts they have. No two people are the same, and so no two people will be helped by the same set of disciplines. Universalizing what is not universal will end up enhancing, instead of undermining, problems. Thus, those who are to guide and direct others should embrace prudence, that is discretion, knowing that through proper discipline, many people can be helped in their spiritual development. This is why the earliest leaders of the desert community, like St. Antony, understood that discretion, not discipline, best served the advancement of virtue:
And so, according to the opinion both of blessed Antony and of all the others, discretion was understood as that which would lead the fearless monk on a steady ascent to God and would always preserve the aforesaid virtues undamaged; as that which the heights of perfection could be scaled with little weariness; and as that without which many of those who labor even with a good will would be unable to arrive at the summit. For discretion is the begetter, guardian, and moderator of all virtues. 
Not everyone has the skill, or grace, to be a spiritual director. It is given to a few. Trying to direct others without it tends to hurt everyone involved, as the one without the gift of discretion will drift to one of two extremes, either legalism, taking the example of what worked for others in the past and trying to use it with everyone, or extreme laxness, which will also hinder proper development, similar to the way a string on a guitar which is too tight will break, or one which is too loose will not make any sound. Not everyone should be giving spiritual advice to others, acting like they are authorities, for those without the gift of discernment, those without wisdom, will tend to make rash judgments. They will engage generalizations and expect everyone to conform to a particular way of life, and those who do not do so, will be mistreated. In acting in such a way, they risk great condemnation, for as James said, those who would take on the role of teacher will be judged much more harshly. Thus, it is better for everyone to not try to take on a role which has not been given to them. Those who find themselves put in positions of authority should engage their authority with due prudence, working hard to develop their own wisdom so that they do not fall into the trap that such authority provides, one which has people try to over-extend themselves and their power by being legalistic and imprudent with their authority, for once they do that, though they might have authority, they are using it to their own spiritual ruin.
 John Cassian, The Conferences 87. [Second Conference; Abba Moses].
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