And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood.
All Commentaries on Exodus 7:20 Go To Exodus 7
Isodore of Seville
AD 636
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–.