" So if you cannot concentrate one of the following is the cause: 1. "Deficiency of the motor centers." 2. "An impulsive and emotional mind." 3. "An untrained mind." The last fault can soon be removed by systematic practice. It is easiest to correct. The impulsive and emotional state of mind can best be corrected by restraining anger, passion and excitement, hatred, strong impulses, intense emotions, fretfulness, etc. It is impossible to concentrate when you are in any of these excited states. These can be naturally decreased by avoiding such food and drinks as have nerve weakening or stimulating influences, or a tendency to stir up the passions, the impulses and the emotions; it is a very good practice to watch and associate with those persons that are steady, calm, controlled and conservative. "

― , The Power of Concentration