On BavaKama 18A: In the case of a chicken picking at a cord to which a bucket is attached, and the cord snapped and the bucket fell and broke the payment is in full.
The Gemara analyses the case and summarizes, after considering Rav Ashis not relying on this Beraita to solve the issue of the nature of Symmachus' ruling regarding payment in full that this beraita is according to the opinion of the Rabbis, and proves that it is the original cause which is the determining factor.
What is faulty with this proof, that it is rejected by Rav Bibi Bar Abaye? Why does he say that the the Beraita talks of a case where The Chicken pushed the Bucket and in the act the bucket broke?
Furthermore - how can he explain thus, when if that is the message of the Beraita - it should not have told us anything about the cord at all, but just about a chicken pushing a bucket?
This is a basic premise in the building of a Sugya. If we want to prove a Safek from a Mishnah or a Beraisa, it is not considered a proof if there is another way to learn the Mishnah. Therefore, Rav Bivi does not have to find any fallacy in the proof.
As for why the cord was mentioned, it was to say that it is the Derech to peck at the cord, for if not it could be Patur for the Keli.
D. Zupnik