As Christians, we should dwell upon, think about, particular categories of things. The 'gatekeeper' of our mind needs to be well-taught and cautiously critical. Paul challenges us to a change of thinking, calculating and analyzing what we meditate upon. New mental pathways need to be formed and followed. Dwell upon what is edifying, properly encouraging to myself and others. If it challenges you to greater morality, godliness and responsibility, then think upon it.