Of what does the difference between purity of mind and purity of heart consist? Purity of mind is different from purity of heart, just as there is a difference between a member of the body and the whole body. The mind is indeed one of the senses of the soul. But the heart is the ruler of the internal senses; that is, the sense of senses, which is the root. If the root is holy, so also are all the branches (c.f. Rom 11:16). But the root is not holy even if in one of the branches there is holiness.
The mind indeed with a little study of the Scriptures and a little labor in fasting and stillness forgets its former musing and is made pure, in that it becomes free from alien habits. It is also easily defiled. The heart, however, is purified with great sufferings and by being deprived of all mingling with the world, together with complete mortification in everything. When it has been purified, however, its purity is not defiled by contact with inconsequential things. That is, it is not afraid of violent battles, for it has a strong stomach which easily digests all foods hard for others who are ill in their abdomens. The purity acquired with great troubles over a long period and by the highest part of the soul does not fear insignificant contacts with worldly things.