Orlah, Chapter Two, Mishnah 12. In the previous mishnah we examined the dispute over the principle of 'zeh vezeh gorem' (this and that cause it together): when a forbidden substance and a permitted substance leaven a mixture together, and neither one would have leavened it on its own - is the result forbidden? The Sages said that it is, while Rabbi Yehoshua said "le'olam ein oser ad sheyehei bo kedei lechametz" - the forbidden substance does not render the mixture forbidden unless it alone contains enough to leaven it, and it cannot rely on the strength of the permitted substance. Our mishnah repeats that very same ruling, and we will see in what form.
The words of the Mishnah:
The mishnah opens: "Yo'ezer ish haBirah hayah mitalmidei Beis Shammai" - Yo'ezer of the Birah was one of the students of Beis Shammai. Who is this 'man of the Birah', and what is the Birah? The commentators differ:
Some say that this is the name of a place near Jerusalem, and Yo'ezer was called after his hometown.
Others say that it refers to a man who was appointed over the Birah.
The identity of the Birah itself is also a subject of dispute in the Jerusalem Talmud in tractate Yoma:
Rabbi Yochanan: a particular place on the Temple Mount was called 'Birah'. Some understand that this was indeed a sort of tower, since 'birah' generally means a palace or a royal city.
Reish Lakish: the entire Beis HaMikdash was called 'Birah', and the reference is to the Temple in general. According to this, Yo'ezer was the man appointed over the Temple.
Yo'ezer's testimony:
Yo'ezer, a student of Beis Shammai, testifies: "sha'alti es Rabban Gamliel haZaken omed besha'ar hamizrach" - he asked Rabban Gamliel the Elder, who was the grandson of Hillel, while he was standing at the eastern gate of the Beis HaMikdash, about this very question: does 'zeh vezeh gorem' render the mixture forbidden or permitted? And his answer was: "le'olam ein oser ad sheyehei bo kedei lechametz" - the forbidden substance does not render the mixture forbidden unless it by itself contains enough to leaven it, and we do not say that the forbidden and the permitted substances together cause the prohibition. So 'zeh vezeh gorem' is permitted.
The inference of the Tosafos Yom Tov:
The Tosafos Yom Tov points out that the mishnah is teaching us something further through this story: Rabban Gamliel was of Beis Hillel, being Hillel's grandson, and Yo'ezer of the Birah was one of the students of Beis Shammai - and even so, Beis Shammai would accept the rulings of Beis Hillel, ask them questions, bring proof from them, and accept their positions as well.
In summary: this mishnah repeats the ruling issued in the previous mishnah - 'zeh vezeh gorem' is permitted, and the forbidden substance does not render the mixture forbidden unless it alone contains enough to leaven it - this time in the name of Rabban Gamliel the Elder at the eastern gate. We examined the identity of Yo'ezer of the Birah, the dispute among the commentators and in the Jerusalem Talmud as to the meaning of 'the Birah', and the inference of the Tosafos Yom Tov that the students of Beis Shammai accepted the rulings of Beis Hillel.