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A person sold a field to his friend, and drew one border long and one border (on the opposite side) short. Rav says: In such a case, the buyer only acquires the amount of field that is opposite the short side.
Rav Kahana and Rav Assi asked Rav: Why shouldn’t the border should be from the small side to the long side (a diagonal line)?
Rav was quiet, and did not answer.
The Chavos Yair (responsa: 152) quotes the Rema who writes that it is well known that Rav is in fact Rav Abba, the chosen disciple of Rabbi Shimon ben Yochai. And everytime that the Gemora says, “Rav was quiet,” it does not mean that he was silent because he did not know; rather, he knew how to answer according to “sod” – the hidden secrets of the Torah, and he did not want to reveal them.
The Chavos Yair explains the Gemora in Bava Kamma (11a), which relates that since Rav Kahana and Rav Assi asked Rav, “Is this truly the halachah?” and he kept quiet, we can conclude that the law of assessment does indeed apply. Although it couls have been said that Rav remained silent for he did not want to reveal the hidden secrets of the Torah, nevertheless, we do not rule in halachic matters based on “sod,” rather, it is solely dependent on the “revealed” portion of the Torah.