There is, no doubt, a double idea in the metaphor which the Psalmist employs. He desires to set forth, by his image of a shadow, not only the transiency, but the unsubstantialness of life. Shadow is opposed to substance, to that which is real, as well as to that which is enduring. And we may further say that the one of these characteristics is in great part the occasion of the other. Because life is fleeting, therefore, in part, it is so hollow and unsatisfying. The fact that men are dragged away from their pursuits so inexorably makes these pursuits seem, to any one who cannot see beyond that fact, trivial and not worth the following. Why should we fret and toil and break our hearts, 'and scorn delights, and live laborious days' for purposes which will last so short a time, and things which we shall so soon have to leave? What is all our bustle and business, when the sad light of that thought falls on it, but 'labouring for the wind'? 'Were it not better to lie still?' Such thoughts have at least a partial truth in them, and are difficult to meet as long as we think only of the facts and results of man's life that we can see with our eyes, and our psalm gives emphatic utterance to them. The word rendered 'walketh' in our text is not merely a synonym for passing through life, but has a very striking meaning. It is an intensive frequentative form of the word--that is, it represents the action as being repeated over and over again. For instance, it might be used to describe the restless motion of a wild beast in a cage, raging from side to side, never still, and never getting any farther for all the racing backward and forward. So here it signifies 'walketh to and fro,' and implies hurry and bustle, continuous effort, habitual unrest. It thus comes to be parallel with the stronger words which follow,--'Surely they are _disquieted_ in vain'; and one reason why all this effort and agitation are purposeless and sad, is because the man who is straining his nerves and wearying his legs is but a shadow in regard to duration--'He heapeth up riches, and knoweth not who shall gather them.'