De Somniis (lib. i-ii)

Thus, therefore, by tracing it out diligently, we have found that praiseworthy speech is likened to a river; but speech which is deserving of blame is the very river of Egypt itself, untractable, unwilling to learn, as one may say in a word, lifeless speech; for which reason it is also changed into blood, [*](Exodus vii. 17. ) as not being able to afford sustenance. For the speech of ignorance is not wholesome, and it is productive of bloodless and lifeless frogs, which utter only a novel and harsh sound, a noise painful to the ear.

And it is said, likewise, that all the fish in that river were destroyed. And by the fish are here figuratively meant the conceptions; for these things float about and exist in speech as in a river, resembling living things and filling the river with life. But in uninstructed speech all conceptions die; for it is not possible to find any thing intelligent in it, but only, as some one has said, some disorderly and unmusical voices of jackdaws.