"A man should conceive of a legitimate purpose in his heart, and set out to
accomplish it. He should make this purpose the centralizing point of his
thoughts."
The purpose is a must for any success, It may be in Spiritual field, Sports, Business or any other area.
Purpose decides, which thoughts is to be strengthen and which thought to be discarded.
If you have clearly decided the purpose of your life, then automatically only purpose related thoughts will stay in your mind.
One should make this purpose his supreme duty and does not allow his thoughts to wander away into ephemeral fancies, longings, and imaginings.