12 “My days have declined like a shadow”....He had said above, “My days are consumed away like smoke;” and he now says, “My days have declined like a shadow.” In this shadow, day must be recognised; in this shadow, light must be discerned; lest afterward it be said in late and fruitless repentance, “What has pride profited us? Or what good has riches with our vaunting brought us? All those things are passed away like a shadow.” Say at this season, all things will pass away like a shadow, and you may not pass away like a shadow. “My days have declined like a shadow, and I am withered like grass.” For he had said above, “my heart is smitten down, and I am withered like grass.” But the grass bedewed with the Saviour's blood will flourish afresh. “I have withered like grass;” I, that is, man, after that disobedience; this I have suffered from Your just judgment: but what are You?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)