Set your mind on (proneite). "Keep on thinking about." It does matter what we think and we are responsible for our thoughts. Not on the things that are upon the earth (mh ta epi th gh). Paul does not mean that we should never think the things upon the earth, but that these should not be our aim, our goal, our master. The Christian has to keep his feet upon the earth, but his head in the heavens. He must be heavenly-minded here on earth and so help to make earth like heaven.