He sends down water from the skies, which flows in channels according to their capacity, with the scum borne on the surface of the torrent, as rises the scum when metals are heated on the fire for making ornaments and household utensils. This is how God determines truth and falsehood. The scum disappears like the foam on the bank, and that which is useful to man remains on the earth. That is how God sets forth precepts of wisdom.