Chapter 1, Mishnah 6, the last in the series of three mishnayos before us, is a direct continuation of the previous mishnah. Rabbi Yehudah does not hold that the explanation given in the previous mishnah for why minority opinions are recorded in the Mishnah is a satisfactory one.
Rabbi Yehudah's difficulty:
He does not dispute the principle established in the previous mishnah, that a third generation cannot overturn the ruling of the second generation if that ruling was based at least on a minority opinion recorded in the Mishnah, unless it is greater than it in wisdom and in number. His claim is that this cannot be the explanation in every single case: sometimes the Mishnah records an individual opinion that there is no possible way to follow, and no one can rely upon it.
When is it impossible to follow the minority opinion?
The commentators take several approaches regarding the circumstances in which there is no way for the Sanhedrin to follow another opinion mentioned in the Mishnah:
A protective measure: Tosafos speak of a type of ruling that was established in a particular generation as a safeguard and a protective measure, and it cannot be challenged in the following generation.
Logical impossibility: Sometimes following that opinion is inconceivable in and of itself.
The example from Mishnah 3:
In Mishnah 3, Hillel said that he had a tradition from Shemayah and Avtalyon regarding the required measure of water, and the weavers came and testified that Shemayah and Avtalyon had said something else, and Hillel retracted. If this is the plain meaning of the mishnah - that Hillel changed his mind and followed the minority tradition of the weavers - then the very position that Hillel originally held no longer exists: there is no one who maintains it, and even Hillel himself abandoned it. It is inconceivable that anyone would follow a position that its own author retracted, and why would a person base his ruling on such a tradition?
From here the question sharpens: what is the point of mentioning Hillel's position, if there is no logical possibility of following it?
The words of the Mishnah:
"Amar Rabbi Yehudah: Im kein, lamah mazkirin divrei hayachid bein hamerubin levatalah?" - if the explanation given earlier is correct, that the entire purpose is to enable one to follow the minority opinion and to lean upon it as a basis and as an additional authority that will not be easily overturned, what is the point of mentioning the minority opinion in a case where there is no possibility of following it? This is the meaning of "levatalah" - where there is no possibility whatsoever of following it in practice.
Rabbi Yehudah's answer:
Rabbi Yehudah answers: "she'im yomar ha'adam: kach ani mekubal" - a person who comes and says that he has a tradition from his teacher. Bear in mind that in the era of the Mishnah there was no Shulchan Aruch; a person had nothing but what he had received as a tradition from his teacher, and that was, by definition, the authority. So this person argues: this is my tradition, this is what my teacher said, and therefore I do not go along with the practice of the Jewish people, because that is not my tradition.
To this, "yomar lo: kedivrei ish peloni shamata" - the Sanhedrin, the Torah authority, responds to him: indeed, you hold a tradition that stems from a genuine line of transmission that exists within the Jewish people. We do not deny that it exists, and the Mishnah even recorded it. But you must know that it was weighed in the Sanhedrin in the days of the Mishnah, and it was rejected in favor of the majority opinion.
The result is that even though he holds a tradition and we accept it - and if he wishes to hold fast to it, he is permitted to follow his own tradition - he may not teach others that the halachah follows him, because the Sanhedrin ruled that the halachah follows the majority, and they rejected his view. His tradition is legitimate and recognized, and it is not a mistake - and that itself is the whole point of mentioning it in the Mishnah: it has a basis, but it is not the halachah, because the Mishnah rejected it.
In summary: in this Mishnah Rabbi Yehudah raised a difficulty on the explanation given in the previous Mishnah: if there are cases in which there is no possibility whatsoever of following the individual opinion - whether because it is a safeguard and a fence that cannot be challenged, or because it is logically impossible, as in a case where the one who held the opinion retracted it himself - why was that opinion taught at all? And his answer is: the individual opinion is recorded so that the one who holds the tradition will know that his tradition does indeed exist, is recognized, and is not a mistake, but rather that it was weighed and rejected in favor of the majority - and therefore he may hold on to it for himself, but he may not rule by it for others.