Psalms 39:6

6.Surely man walketh in a shadow. (69) He still prosecutes the same subject. By the word shadow, he means, that there is nothing substantial in man, but that he is only, as we say, a vain show, and has I know not how much of display and ostentation. (70) Some translate the word darkness, and understand the Psalmist’s language in this sense, That the life of man vanishes away before it can be known. But in these words David simply declares of every man individually what Paul extends to the whole world, when he says,

“The fashion of this world passeth away.” —
1 Corinthians 7:31

Thus he denies that there is any thing abiding in men, because the appearance of strength which displays itself in them for a time soon passes away. What he adds, that men disquiet themselves in vain, shows the very height of their vanity; as if he had said, It seems as if men were born for the very purpose of rendering themselves more and more contemptible: for although they are only as a shadow, yet as if they were fools, or rather insane, they involve themselves needlessly in harassing cares, and vexing themselves to no purpose. He expresses still more plainly how they manifest their folly, when he declares that while they anxiously and carefully heap up riches, they never think that they must soon, and it may be suddenly, leave their present abode. And why is it that they thus fret away their mind and body, but only because they imagine that they can never have enough? for by their insatiable desire of gain, they eagerly grasp at all the riches of the world, as if they had to live a hundred times the life of man. Moreover, David does not in this passage hold up to scorn the covetousness of man in the same sense in which Solomon does, Ecclesiastes 5:10; for he not only speaks of their heirs, but declares generally, that men disquiet and vex themselves with care, although they know not who shall reap the fruit of their labor in amassing riches. (71) They may indeed wish to make provision for themselves; but what madness and folly is it for them to torment themselves with incessant and unprofitable cares which have no certain object or limit? David here condemns those ardent and unbridled desires, under the influence of which worldly men are carried away, and talk in a strange manner, confounding heaven and earth; for they admit not that they are mortal, much less do they consider that their life is bounded by the narrow limits of a hand-breadth. David spoke under the influence of a distempered and troubled state of mind; but there is included in his language this very profitable lesson, that there is no remedy better fitted for enabling us to rise above all unnecessary cares, than the recollection that the brief period of our life is only, as it were, a hand-breadth.