Chapter 4, Mishna 4
We have a series of principles about whom the law follows in certain disputes. For example, when Rabbi Yehuda disagrees with Rabbi Shimon the law follows Rabbi Yehuda. Rav Mesharshiya holds that these principles are not valid. The Gemara examines a series of unrelated laws in which either the law accords with the Sage who’s should lose according to these principles, OR the law accords with the correct sage, and the Gemara asks why it needed to be spelled out, given that we have these principles.
The Gemara finally concludes that the principles are valid, but not all the sages subscribe to them, including the great sage Rav.