Discussions about the spiritual life in modern times rarely seem to give attention to purity of mind and heart. We expose ourselves indiscriminately to a plethora of distractions - each strengthen the grip of the passions upon us. And even when attention is given to the subject it is typically amorphous - lacking the clarity and structure of the Fathers' thought. Yet without such attention given to the interior life men and women are left to wander aimlessly - seeking to fill the void with various popular spiritual practices but never coming to know the healing needed and desired.
St. Isaac the Syrian in the following brief summary captures the essence of the struggle: If the mind is made pure through guarding the senses through ascetical practices like fasting, vigils and the study of scriptures, one can begin to know a surface level purity - the mind is cleansed of former thoughts and the imagination and memory are gradually made whole. Such purity, he warns, may be short lived if due diligence is not maintained. The mind will be quickly soiled again. As the author of Proverbs writes: "As a dog returns to his vomit, so the sinner returns to his sin."
The purification of the heart, Isaac goes on to explain, only comes through affliction - where attraction to the things of the world is lost and one becomes, as it were, dead to all thing or as St. Paul wrote: "It is no longer I who live I, but Christ who lives within me." One puts on the mind of Christ. Few are willing to walk this narrow and difficult path or have the stomach to endure the dreadful conflicts and trials that produce such freedom. When one clings only to God and his will, the assaults of the world endanger but a little.
The path put forward seems so simple but it is avoided and ignored but those ruled by their passions and desires for the things of the world. May God give us the eyes to see and the ears to hear. . . .
Purity of mind is one thing, and purity of heart is another, just as a limb differs from the whole body. Now the mind is one of the senses of the soul, but the heart is what contains and holds the inner senses: it is the sense of senses, that is, their root; but if the root is holy, then the branches are holy. It is evident, therefore, that if the heart is purified, all the senses are made pure. Now if the mind, on the one hand, is a little diligent in reading the divine scriptures and toils a little in fasting, vigil, and stillness, it will forget its former activity and will become pure, as long as it abstains from alien concerns. Even so its purity will not be permanent, for just as it is quickly cleansed, so too it is quickly soiled.
But the heart, on the other hand, is only made pure by many affliction, deprivations, separation from all fellowship with the world, and deadness to all things. Once it is purified, however, its purity is not soiled by little things, nor is it dismayed by great and open conflicts (I mean dreadful ones), inasmuch as it has acquired, as it were, a strong stomach capable of quickly digesting all the food that is indigestible to those who are weak. For so it is said among the physicians, that all meats difficult to digest, but it produces great strength in healthy bodies when a strong stomach takes it.
Even so, any purity that comes quickly, with little time and slight labor, is also quickly lost and defiled. But the purity that comes through many afflictions and is acquired over a long period of time in the soul's superior part is not endangered by any moderate assault.