The mind is man’s greatest and only treasure. God has fashioned this mind in such a way that it will always be subservient to you. If the mind were disobedient by nature, we would not have been responsible for any of our actions. In that case, man would neither be free agent nor would be counted as the acme of creation. You are the complete master of your mind and you can shape it as you like. When the mind is completely within our control, it will have nothing to exist on except noble thoughts. For the nourishment of the body we are to give it only wholesome food avoiding unwholesome things: similarly, we are to nourish the mind with noble thoughts, high ideals, and good discourses refusing to provide it with unwholesome food like evil thoughts and bad associations.
The greatest duty of man is to keep his mind centered on God. With every breath, we should think of Him.
Reference: Swami Vijnanananda – Life And Teachings p55-564