John Cassian, Conferences XI–XVII XIV.3: How practical perfection depends on a double system

But this practical perfection depends on a double system; for its first method is to know the nature of all faults and the manner of their cure. Its second, to discover the order of the virtues, and form our mind by their perfection so that it may be obedient to them, not as if it were forced and subject to some fierce sway, but as if it delighted in its natural good, and throve upon it, and mounted by that steep and narrow way with real pleasure. For in what way will one, who has neither succeeded in understanding the nature of his own faults, nor tried to eradicate them, be able to gain an understanding of virtues, which is the second stage of practical training, or the mysteries of spiritual and heavenly things, which exist in the higher stage of theoretical knowledge? For it will necessarily be maintained that he cannot advance to more lofty heights who has not surmounted the lower ones, and much less will he be able to grasp those things that are without, who has not succeeded in understanding what is within his comprehension. But you should know that we must make an effort with a twofold purpose in our exertion; both for the expulsion of vice, and for the attainment of virtue. And this we do not gather from our own conjecture, but are taught by the words of Him who alone knows the strength and method of His work: “Behold,” He says: “I have set thee this day over the nations and over kingdoms, to root up, and to pull down, and to waste, and to destroy, and to build and to plant.” He points out that for getting rid of noxious things four things are requisite; viz., to root up, to pull down, to waste, and to destroy: but for the performance of what is good, and the acquisition of what pertains to righteousness only to build and to plant. Whence it is perfectly evident that it is a harder thing to tear up and eradicate the inveterate passions of body and soul than to introduce and plant spiritual virtues.