The first thing in purifying the mind is to be able to discern the foreign element there. As all that is foreign to the body does not agree with the body, making it ill, so all that is foreign to the mind disturbs the peace of the mind, and it is that which proves that it does not belong to the mind. Such things are worry, anxiety, fear, sorrow, or any sort of disturbance that takes away the tranquility of the mind, preventing it from experiencing that joy and peace for which it longs and in which alone is its satisfaction.