1 The Psalm which we have just heard chanted, and have responded to with our voices, is short, and very profitable. You will not long toil in hearing, nor will you toil fruitlessly in working. For it is, according to the title prefixed to it, “A song of degrees.” Degrees are either of ascent or of descent. But degrees, as they are used in this Psalm, are of ascending...There are therefore both those who ascend and those who descend on that ladder. Who are they that ascend? They who progress towards the understanding of things spiritual. Who are they that descend? They who, although, as far as men may, they enjoy the comprehension of things spiritual: nevertheless, descend unto the infants, to say to them such things as they can receive, so that, after being nourished with milk, they may become fitted and strong enough to take spiritual meat...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)