Saint Hesychius writes,
One who does not have prayer that is free from the thoughts is without a weapon for battle. I understand prayer to be that which is carried out unceasingly within the depths of the soul, so that the enemy who is secretly fighting may be vanquished and scorched by this invocation to Christ. For you must look with the sharply focused eye of the mind so that you will recognize what has entered into it, and after doing so, immediately cut off the head of the snake through refutation, and at the same time call on Christ with groaning. through experience you will come to know God's invisible help; then you will see clearly the true condition of the heart.Our challenge is to have the sobriety to be watchful and attentive to the nature of our thoughts and to dismiss them when we enter into prayer. As Saint Hesychius says, "you must look with the sharply focused eye of the mind" in prayer. This is the true purpose of our mind to enable us to focus on our Creator and discern His will for us. As we repeat the Jesus Prayer, our mind will instantly become focused on God and we will receive His grace which helps us deter all distracting thoughts.
One of the reasons we pray is to overcome the domination of our soul by the thoughts roaming thorough our minds, distracting it from a intimate relationship with God. It is in prayer where thoughts are subdued that we find this relationship our soul seeks.
More on Jesus Prayer
Reference: The Spiritual Life, p 252