25 This I say, brethren, that you may profit from what you have heard, and ruminate within yourselves: permit not yourselves to forget, not only by thinking over again upon these subjects, and discoursing upon them, but also by so living. For a good life which is led after God's commands, is like a pen, because it is heard writing in our hearts. If it were written on wax, it would easily be blotted out: write it in your hearts, in your character, and it shall never be blotted out.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)