"...We Orthodox think of prayer as growing more and more inward. It begins as prayer of the lips—oral prayer—but then it grows more inward—prayer of the mind. Because prayer which is merely said with the lips is not true prayer. We need to pray with our mind, with our inner attention, so that it becomes prayer of the mind.
But then we say that there is a further stage where the prayer can descend from the mind into the heart. When Eastern Christian writers speak of the heart, they do not just mean the emotions and feelings. The heart signifies the spiritual center of the total human person. The heart is the place where we make model decisions. The heart is the seat of understanding and wisdom. It is in the heart that we know ourselves as made in the image and likeness of God.
This understanding of the heart, not just as emotions and feelings, but as the spiritual center of the total person, is exactly the way that it is understood in scripture. So when we speak of the prayer descending from the mind into the heart, we mean that the prayer is to be identified with our total being. It is not to be just something that we think about with our minds in a detached way. The prayer is to become, not just something that we do, but something that we are. Prayer of the heart means prayer of the total person in which the reasoning brain, the emotions, the feelings, the deep understanding, yes, and also the body—all of these are to participate in prayer. So, this is what we mean by descending upon the mind and into the heart. The prayer becomes, by God’s grace and not just by our own efforts, prayer of the whole of ourselves."
- Metropolitan Kallistos Ware