Vv. 15-18 tell how the psalmist strove with and finally conquered his doubts, and saw enough of the great arc of the Divine dealings, to be sure that the anomaly, which had exercised his faith, was capable of complete reconciliation with the righteousness of Providence. It is instructive to note that he silenced his doubts, out of regard to "the generation of Thy children"--that is, to the true Israel, the pure in heart. He was tempted to speak as others did not fear to speak, impugning God's justice and proclaiming the uselessness of purity; but he locked his lips, lest his words should prove him untrue to the consideration which he owed to meek and simple hearts, who knew nothing of the speculative difficulties torturing him. He does not say that his speaking would have been sin against God. It would not have been so, if, in speaking, he had longed for confirmation of his wavering faith. But whatever the motive of his words, they might have shaken some lowly believers. Therefore he resolved on silence. Like all wise and devout men, he swallowed his own smoke, and let the process of doubting go on to its end of certainty, one way or another, before he spoke. This psalm, in which he tells how he overcame them, is his first acknowledgment that he had had these temptations to cast away his confidence. Fermentation should be done in the dark. When the process is finished, and the product is clear, it is fit to be produced and drank. Certitudes are meant to be uttered; doubts are meant to be struggled with. The psalmist has set an example which many men need to ponder to-day. It is easy, and it is also cruel, to raise questions which the proposer is not ready to answer. Silent brooding over his problem did not bring light, as ver. 16 tells us. The more he thought over it, the more insoluble did it seem to him. There are chambers which the key of thinking will not open. Unwelcome as the lesson is, we have to learn that every lock will not yield to even prolonged and strenuous investigation. The lamp of the Understanding throws its beams far, but there are depths of darkness too deep and dark for them; and they are wisest who know its limits and do not try to use it in regions where it is useless.