Attention to Breathing (ānāpāna sati) 

Attention to breathing is commonly practiced today, as an exercise in concentration, but as it was taught by the Buddha it was not an exercise in concentration. It was an exercise in relaxation. By learning to relax the breathing, one can relax the entire body. When the body is relaxed, the mind calms down. When the mind is calmed, the mind and body are free from emotional agitation. As emotional disturbance disappears, the mind attains a state of equilibrium (samādhi). Samādhi is not concentration as commonly translated, it is equilibrium of the mind and body (sama = equal; dhi = the state). It is a state of mental and physical balance, or stillness of mind and body. Concentration implies tension and effort, whereas equilibrium is a state of rest and relaxation. 
The purpose of Samādhi is to rest the mind or stop the activity called mind. Mind is not an entity different from the body. It is merely an activity of the body, which is mainly the activity of the nervous system, consisting of the brain, the spinal cord, the nerves and the senses. In fact, the entire body is involved in this activity called mind, just as the entire engine is involved in its activity. 


Samādhi is a gradual stopping of this activity called mind, until it comes to an entire stop. This was what the Buddha achieved. After having stopped the mind, he restarted it. It was when he restarted it that he awoke from the dream of existence, and thus became a Buddha. Details of this will be discussed later but at this point we only need to know that what we call the mind is just an activity of the body. Therefore achieving tranquility of mind is nothing but stilling the activity.