You know that our breathing is the inhaling and exhaling of air. The organ that serves for this is the lungs that lie round the heart, so that the air passing through them thereby envelops the heart. Thus breathing is a natural way to the heart. And so, having collected your mind within you, lead it into the channel of breathing through which air reaches the heart and, together with this inhaled air, force your mind to descend into the heart and to remain there.
Nicephorus the Solitary