Job 15:31

In vanity, i.e. in the vain and deceitful things of this world, such as power, riches, honour, &c., of which, and of the loss of them, he had been largely discoursing; and now he subjoins a general caution to all men to take heed of running into the same error and mischief with the forementioned persons; and withal he secretly intimates Job's great sin, which was the cause of his ruin, to wit, his carnal confidence in those vain things, the wealth, and glory, and children which God had given him; from which crime he therefore vindicates himself hereafter. For vanity, i.e. disappointment and dissatisfaction, and the vanishing or loss of all their imaginary felicity; the same word vanity being elegantly repeated in another sense, as is usual in Scripture and all authors. His recompence, Heb. his exchange; he shall exchange one vanity for another, a pleasing vanity for a vexatious vanity. But this verse is and may be rendered otherwise; the Hebrew particle al being here, as it is elsewhere, taken for a simple negation. Thus, He that is deceived with vanity, (i.e. with the vain things of this world, wherewith most men are deceived and bewitched,) will not believe that vanity (i.e. emptiness, and disappointment, and misery) shall be his recompence. And so this is an aggravation of his calamity, that it surprised him when he was confident and secure from all fears of such an event.