1....The Psalm which we have just sung is in many parts somewhat obscure. When by the help of the Lord what has been said shall begin to be expounded and explained, you will see that you are hearing things which you knew already. But for this cause are they said in manifold ways, that variety of expression may remove all weariness of the truth....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)