The Third Law is Purpose.
Without purpose, we are no different than animals: instinctively roaming about, completely at the whim of the Dark World.
Purpose is something we can change. Purpose is something we can decide for ourselves. We are not bound to mere survival, as are the beasts of the Earth. We have possibilities. Without Purpose, there is no point to accomplishing anything.
What is your life's purpose? Do you even know?
Don't be surprised if you can't explicitly answer this. I couldn't either, for a very long time.
At some point you're probably going to stop and think, "What am I doing with my life?" which is code for "I don't know what my life's purpose is." And if you're a Christian, the answer isn't necessarily simplified. As a young believer in the faith, my thoughts went something like this:
"What is my purpose? To do whatever God wants me to do. My purpose is to… to what? Provide for my family? Glorify God? Those are all pat answers. I want to know what my singular, most powerful purpose really is."
But what does that mean, exactly? What does it mean to "fulfill God's purpose in your life?" Can you give specifics? Can you quantify "Love thy neighbor as yourself?" Do these look the same for every person? Of course they don't. Over the years, God has given people purpose in many ways, for many different reasons. And He's even changed people's purposes over time. So again, what does that mean for you and me?