from 'Life of Moses'
And many times the master of deceit endeavors to turn the drink of the Hebrews also into blood by polluting it with falsehood that is by presenting our doctrine to us as different as what it really is. But he cannot make the water wholly unusable, even if he should easily turn it red by his trickery. For since he pays no attention to the optical illusion, the Hebrew drinks the true water, even though he is successfully misled by his adversaries.
The same applies to the frogs - ugly and noisy amphibians, leaping about not only unpleasant to the sight, but also having a foul-smelling skin – they entered the houses, the beds, the storerooms of the Egyptians, but they did not affect the life of the Hebrews
One sees in the sordid and licentious life that which is indeed born out of clay and mire, and that which through imitation of the irrational remains in a form of life neither altogether human nor frog. Being a man by nature and becoming a beast by passion, this kind of person exhibits an amphibious form of life ambiguous in nature. In addition, one will find the evidences of such an illness, not only on the bed but also on the table and in the storeroom and throughout the house.