Cassiodorus : Just as we read in the Gospel that water was turned into wine, which denoted that people were changed for the better, so here its transformation into blood announces that sinners interpret the causes of spiritual things in a bodily sense. Blood is introduced here to denote the flesh, and undoubtedly the Jewish people took this materialistic view. He further says that both their rivers and their rain showers were turned into blood, so that in their preoccupation with the thoughts of the flesh they did not understand the heavenly preaching in a spiritual sense. The literal sense of this and of what follows is clear, for the words of the divine history show that these events occurred in Egypt. — EXPOSITION OF THE Psalms 77.44 Isidore of Seville : Then the plagues are visited upon Egypt. They were carried out corporally among the Egyptians; they are now carried out spiritually in us, for Egypt is the figure of this world. The first plague is the one in which the waters are turned into blood. The waters of Egypt are erratic, just as the dogmas of the philosophers are inconstant. These waters are deservedly turned into blood, because when the philosophers ponder the causes of things they think carnally. But when the cross of Christ shows the light of truth to this world, it will reproach the world with censures of this sort, so that from the kind of punishment it suffers, the world might recognize its errors. — QUESTIONS ON THE OLD TESTAMENT, Exodus 14:1-2