Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things. Philippians 4:8

In the first chapter of Philippians, Paul prays for his friends that their love would grow in knowledge and discernment so they could better understand and act in ways that bring God joy and praise. Here at the end of his letter, Paul is giving his friends a practical tip for HOW one is able to do just that. He says be disciplined in your thought life.