5 “Man is made like vanity: his days pass away like a shadow”. What vanity? Time, which passes on, and flows by. For this “vanity” is said in comparison of the Truth, which ever abides, and never fails: for it too is a work of His Hand, in its degree. “For,” as it is written, “God filled the earth with His good things.” What is “His”? That accord with Him. But all these things, being earthly, fleeting, transitory, if they be compared to that Truth, where it is said, “I Am That I Am,” all this which passes away is called “vanity.” For through time it vanishes, like stroke into the air. And why should I say more than that which the Apostle James said, willing to bring down proud men to humility, “What is,” says he, “your life? It is even a vapour, which appears for a little time, and then vanishes away.”...Work then, though it be in the night, with your hands, that is, by good works seek God, before the day come which shall gladden you, lest the day come which shall sadden you. For see how safely you work, who art not left by Him whom you seek, “that your Father which sees in secret may reward you openly.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)