The following quotation is from the book The Secret of Success by Napoleon Hill. He studied 500 of the most successful people in the world for years to come up with the ideas for his book, which was a best seller in its day.
“Self-control is solely a matter of thought-control.
Please read the above sentence aloud; read it thoughtfully and meditate over it before reading further, because it is, without doubt, the most important single sentence of this entire course.
You are studying this course presumably because you are earnestly seeking truth and understanding sufficient to enable you to attain some high station in life.
You are searching for the magic key that will unlock the door to the source of power; and yet you have the key in your own hands, and you may make use of it the moment you learn to control your thoughts.
Place in your own mind, through the principle of auto-suggestion, the positive, constructive thoughts which harmonize with your definite chief aim in life, and that mind will transform those thoughts into physical reality and hand them back to you, as a finished product.
This is thought-control!
When you deliberately choose the thoughts which dominate your mind and firmly refuse admittance to outside suggestion, you are exercising self-control in its highest and most efficient form. Man is the only living animal that can do this.”