"Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things." (Phil. 4:8)
"Think on these things" meaning to 'take inventory' (Strong). This deals with what Christians are suppose to reflect upon, or to 'meditate upon for the purpose of putting it into practice' (Gill). While we all have the freedom to think individually, (and thank God He did not make us all to think exactly alike - makes the world a little bit more interesting - at least that's what I think) we are responsible before God to make sure that we don't just entertain every thought that comes to mind. Every Christian is commanded to truly think the kind of thoughts that are pleasing to God. We are given some objectives to reflect upon. The reason why 'these things' are God-pleasing is because they represent or rather they are consistent with who God really is. In other words, 'these things' reminds us about God's character. And so in keeping them in mind, we really are keeping God in mind. "These things" are to help us aspire to do great things for our great God, in a manner that is godly.