6 Next, the man of God, or rather the Prophetic spirit, seems to be reciting some law written in the secret wisdom of God, in which He has fixed a limit to the sinful life of mortals, and determined the troubles of mortality, in the following words: “Their years are as things which are nothing worth: in the morning let it fade away like the grass”. The happiness therefore of the heirs of the old covenant, which they asked of the Lord their God as a great boon, attained to receive this Law in His mysterious Providence.
Moses seems to be reciting it: “Their years shall be things which are esteemed as nothing.” Such are those things which are not before they have come: and when come, shall soon not be: for they do not come to be here, but to be gone. “In the morning,” that is, before they come, “as a heat let it pass by;” but “in the evening,” it means after they come, “let it fall, and be dried up, and withered”. It is “to fall” in death, be “dried up” in the corpse, “withered” in the dust. What is this but flesh, wherein is the accursed lust of fleshly things? “For all flesh is grass, and all the goodliness of man as the flower of the field; the grass withers, the flower fades: but the word of the Lord abides for ever.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)