פירוש על עבודה זרה 74:14

This baraita demonstrates that Yose b. Yo’ezer took efforts to define the public and private domains. The assumption is that this was in order to delineate when doubtful cases of impurity are treated as impure—in the public domain they are not, and in the private domain they are.

According to Rashi people used to come in front of R. Yannai and ask him about cases of doubtful impurity in the public domain. He would tell them to go immerse in the deep waters of the river, as if to say, “what’s so hard about that?”