18 “For He knows our forming”: that is, our infirmity. He knows what He has created, how it has fallen, how it may be repaired, how it may be adopted, how it may be enriched. Behold, we are made of clay: “The first man is of the earth, earthy: the second man is the Lord from heaven.” He sent even His own Son, Him who was made the second man, Him who was God before all things. For He was second in His coming, first in His returning: He died after many, He arose before all. “He knows our forming.” What forming? Ourselves. Why do you say that He knows? Because He has pitied. “Remember that we are but dust.” Addressing God Himself, he says, “Remember,” as if God could forget: He perceives, He knows in such a manner that He cannot forget. But what means, “Remember”? Let your mercy continue towards us. You know our forming; forget not our forming, lest we forget your grace.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)