De Somniis (lib. i-ii)

again, they are of use inasmuch as the one irrigates the fields, and the other fertilizes the souls of those who receive it. And at times they are injurious by reason of overflowing, as then the one deluges the land on its borders, and the other troubles and confuses the reason of those who do not attend to it.

Therefore speech is compared to a river, and the nature of speech is twofold, the one sort being better and the other worse; that is, the better kind which does good, and that of necessity is the worse kind which does harm;