…he who has not been able to understand the things in his own heart, will be much less able to grasp what is outside him.
We must know however, that we are to toil with a twofold end in view, in casting out our faults and in acquiring virtue. And this we hold, not by any guess of our own, but are instructed by the word of Him Who alone knows what His handiwork is, and can do. “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” (Jeremiah 1:10). For, in casting out hurtful things He shows us four necessities–rooting out, pulling down, dispersing, and destroying; but in the practice of virtue and the attainment of what has to do with righteousness, we need only build and plant.
Hence it is very clear that it is more difficult for passions of body and soul which have become inveterate, to be torn up and rooted out, than for spiritual virtues to be reared and planted.
~St John Cassian (Conference with Abbot Nesteros)
Prayerful Life 