This is referred to in the Bible as the Law of Sowing and Reaping which says that, "Whatsoever a man soweth, that also shall he reap."

Sir Isaac Newton called it the third principle of motion. He said, "For every action, there is an equal and opposite reaction." 

For you and I, the most important expression of this universal law is that, "Thoughts are causes and conditions are effects." 

Put another way, "Thought is creative." Your thoughts are the primary creative forces in your life. You create your entire world by the way you think. All the people and situations of your life have only the meaning you give them by the way you think about them.  And when you change your thinking, you change your life, some times in seconds!

The most important principle of personal or business success is simply this: You become what you think about most of the time.

It is not what happens to you but how you think about what happens to you that determines how you feel and react. It is not the world outside of you that dictates your circumstances or conditions. It is the world inside you that creates the conditions of your life.