EVERYTHING should be done and sought after by us for the sake of this [purity of heart]. For this we must seek for solitude, for this we know that we ought to submit to fastings, vigils, toils, bodily nakedness, reading, and all other virtues that through them we may be enabled to prepare our heart and to keep it unharmed by all evil passions, and resting on these steps to mount to the perfection of charity, and with regard to these observances, if by accident we have been employed in some good and useful occupation and have been unable to carry out our customary discipline, we should not be overcome by vexation or anger, or passion, with the object of overcoming which, we were going to do that which we have omitted. For the gain from fasting will not balance the loss from anger, nor is the profit from reading so great as the harm which results from despising a brother. Those things which are of secondary importance, such as fastings, vigils, withdrawal from the world, meditation on Scripture, we ought to practise with a view to our main object, i.e., purity of heart, which is charity, and we ought not on their account to drive away this main virtue, for as long as it is still found in us intact and unharmed, we shall not be hurt if any of the things which are of secondary importance are necessarily omitted; since it will not be of the slightest use to have done everything, if this main reason of which we have spoken be removed, for the sake of which everything is to be done. For on this account one is anxious to secure and provide for one's self the implements for any branch of work, not simply to possess them to no purpose, nor as if one made the profit and advantage, which is looked for from them, to consist in the bare fact of possession but that by using them, one may effectually secure practical knowledge and the end of that particular art of which they are auxiliaries. Therefore fastings, vigils, meditation on the Scriptures, self-denial, and the abnegation of all possessions are not perfection, but aids to perfection: because the end of that science does not lie in these, but by means of these we arrive at the end. He then will practise these exercises to no purpose, who is contented with these as if they were the highest good, and has fixed the purpose of his heart simply on them, and does not extend his efforts towards reaching the end, on account of which these should be sought: for he possesses indeed the implements of his art, but is ignorant of the end, in which all that is valuable resides. Whatever then can disturb that purity and peace of mind--even though it may seem useful and valuable--should be shunned as really hurtful, for by this rule we shall succeed in escaping harm from mistakes and vagaries, and make straight for the desired end and reach it.
John Cassian, Conferences, Chapter 7