Here, the Almighty calls attention to His manifest law in the form of an oath as testimony for the truth of the revelation of the Quran. It is plainly seen in the laws of Nature that rain comes down from heaven in time of need. The greenness and verdure of the earth depend upon heavenly rain. If it ceases for a time, the water in the upper strata of the earth gradually dries up. Thus we perceive that there is a connection between the heavenly and the earthly waters.
Revelation stands in the same relation to human reason as heavenly water does to the earthly water.
As with the cessation of heavenly water earthly water begins gradually to dry up, so also is the case with human reason which, without the heavenly revelation, loses its purity and strength. When, therefore, a long period of time elapses without witnessing the appearance of any inspired one of God, it is but natural that reason, like earthly water, should become noxious and impure. As an illustration of this principle, we may cast a glance at the pre-Islamic period. Before Prophet Muhammad had appeared, darkness prevailed everywhere. About six hundred years had passed since the advent of Jesus Christ, and during this long interval no inspired one of God had made his appearance.