During our study of Orlah we have learnt some laws that more broadly relate to mixtures involving prohibited ingredients. For example we have learnt that if a mixture contains a prohibited leavening agent (e.g. kelei kerem sourdough mixed into regular dough) then the mixture is always prohibited irrespective of the ratio of permitted ingredients to the prohibited ones (2:4)1. A debate ensues in that Mishnah regarding whether the fact that the prohibition is not annulled also has implications for the laws of tumah as well.*Ordinarily, for tameh foodstuff to transfer tumah it needs to at least be the size of a kebeitza. Beit Hillel maintains that in our case as well this requirement is maintained. Beit Shammai however argue, since the prohibited sourdough is not annulled, even if it is less than a kebeitza, the entire mixture is tameh*. Let us try and understand this debate.
The Mishnah Rishona explains that Beit Shammai understood that the requirement of a kebeitza for tumah transfer is because this size is substantial – it is significant (chashuv) (Pesachim 49b). In our case, since the sourdough is not batel (annulled) it should also be consider significant and therefore transfer tumah to the bread.
Beit Hillel however would argue in one of two ways. Either that the reason for a kebeitza is not because of significance, but rather it was the shiur that was handed down halacha le’moshe mi’sinai. Consequently not being batel is irrelevant as a kebeitza is always required. Alternatively Beit Hillel might agree with everything that Beit Shammai claim. Nevertheless, since the law that the leavening agent is rabbinic, the Chachamim had no desire to increase tumah due to this rabbinic law and therefore excluded tumah from it (see Chulin 1:2).
Note that according to the Mishnah Rishona, in Beit Shammai’s view the sourdough is considered as if it is a kebeitzah. The tumah is therefore transferred from the sourdough to the rest of the dough. If that were the case it would seem that the rest of the dough would have had to undergo hechsher. (For any food to become susceptible to tumah it must first come into contact with one of the seven liquids. This is referred to as hechsher.)
The Rashash however feels that it is obvious that according to Beit Shammai the dough does not require hechsher.How then should we understand the position of Beit Shammai according to the Rashash?
Perhaps the answer is found in the Rashash’s own doubt regarding whether the dough would require hechsher according to Beit Hillel if a kebeitzah of dough was mixed in. He explains that either Beit Hillel understands that the tumah is transferred and the dough would thus require hechsher. This is much our understanding of the Mishnah Rishona’s explanation of Beit Shammai above. Alternatively, the Rashash explains that “even without hechsher, the sourdough could cause it to be tameh since it was leavened because of it”. To explain, since the sourdough affected the dough (and is not batel) it makes the entire mixture into one tameh body.2
As a postscript, the next Mishnah records that Dustai heard from Shammai that he held the same opinion as Beit Hillel. The Bartenura writes, “And thus is the Halacha.” The Shoshanim Le’David questions the necessity of this Mishnah and the Bartenura’s comment; we rule like Beit Hillel against Beit Shammai anyway!
When there is a debate between and Beit Shammai and Beit Hillel, we do not rule like Beit Hillel because Beit Shammai is wrong. “Elu ve’elu diver elokim chayim” -they are both really considered correct; it is just that Beit Hillel merited having the practical Halacha according to their opinion. Dustai comments were therefore necessary for the sake of truth. Shammai agreed with Beit Hillel and Beit Shammai’s position was in error.
1 Note that this law only applies when the issur and heter are of the same type (min b’mino). R’ Akiva Eiger notes that sourdough tastes differently and is referred to by a different name than dough. Consequently, one might think that the law should not apply in such a case. Nevertheless, he explains, since dough can become sourdough it is considered min b’mino.
2 Perhaps this is much like the case in Keilim (18:7) where a bed leg that is tameh midras and is attached to a bed makes the entire bed tameh midras while it remains attached.
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