The word translated here as "just" (it's also frequently translated "right") is "dikaios" and the short definition is "just, righteous, impartial." The implication is that in every interaction and every relationship, our thoughts should come in line with God's thoughts.
I'm loving the way Paul is layering his words. It's impossible to think about things that are "just" without knowing Truth. The more you think about true and honorable things, the more your thoughts will begin to line up with God's thoughts, and the more you'll be able to think about things that are just and righteous.
In Matthew, Jesus says, "Blessed are those who hunger and thirst for righteousness, for they shall be satisfied."
How hungry are you?