1) CONSIDERING SEPARATE ACTS OF EATING AS A SINGLE ACT
OPINIONS: In the Mishnah (12b), Rebbi Meir and the Chachamim argue about how long one may delay while eating a forbidden food and still be liable. Rebbi Meir maintains that he may delay for as long as it takes a person to eat finely-ground crumbs. The Chachamim maintain that he is liable only if the delay from beginning to end was less than Kedei Achilas Peras (within the time needed to eat half a loaf).
There are two basic ways to understand the Machlokes in the Mishnah.
(a) RASHI (DH l'Chumra) explains that Rebbi Meir's words, "k'Ilu Achal Kelayos," is a description of a length of time. It is the amount of time that it takes to eat one Peras (two k'Zeisim) of finely-ground crumbs. If a person ate the Peras of Isur within the time span of "k'Ilu Achal Kelayos," he is Chayav even if he paused between the eating of the two k'Zeisim.
The Chachamim argue and maintain that only when he eats the two k'Zeisim within Kedei Achilas Peras is he Chayav.
(b) The RAMBAM (in Perush ha'Mishnayos) explains that Rebbi Meir is describing a type of eating, and not an amount of time for eating. If the offender ate the two k'Zeisim continuously -- even if he took from morning until evening, then he is Chayav. If, however, he paused in his act of eating, then Rebbi Meir agrees with the Chachamim that the two k'Zeisim must be eaten within Kedei Achilas Peras in order for him to be Chayav. (That is, Rebbi Meir takes into account the time of the actual chewing. The Chachamim require that the time from the beginning of the first k'Zayis until the end of the second k'Zayis take no more than Kedei Achilas Peras, including the time that it takes to chew them.)
2) PERMITTING A PREGNANT WOMAN TO EAT A PARTIAL "SHI'UR"
QUESTION: The Gemara quotes a Beraisa that says that the Rabanan permitted a pregnant woman to eat a partial Shi'ur of Tum'ah on account of danger (her health will be at risk if she does not eat food that she craves).
Why does the Beraisa need to give a special allowance for a pregnant woman to eat Tum'ah in order to avoid danger to her life? Even though eating more than a Shi'ur will make her Tamei, there is no prohibition against eating Tamei foods and becoming Tamei!
(a) TOSFOS (DH Hitiru) answers that the Beraisa refers to a case in which the pregnant woman must eat Terumah after she eats the Tamei food. One who is Tamei is certainly forbidden to eat Terumah, but since the pregnant woman will eat only a partial Shi'ur of Tum'ah, she may eat Terumah afterwards. If not for the danger to her health, she would not be permitted to eat a partial Shi'ur of Tum'ah and then eat Terumah, due to an Isur d'Rabanan that prohibits one from eating Terumah after he has eaten even less than a Shi'ur of Tum'ah. However, since she craves for this Tamei food immediately, we may give it to her, even though she will eat Terumah afterwards. (See SEFAS EMES.)
(b) RABEINU GERSHOM understands that the Beraisa is discussing an entirely different case. According to Rabeinu Gershom, the Beraisa refers to a pregnant woman who smells food on Yom Kippur and craves the food. Due to the concern for Piku'ach Nefesh, she is permitted to eat the food on Yom Kippur. (It is not clear why this Halachah is mentioned in the middle of a Beraisa which discusses the laws of Tum'ah and Taharah.)
How does Rabeinu Gershom explain the continuation of the Gemara? The Gemara continues and asks that if she is in danger, then she should be permitted to eat even more than just a partial Shi'ur. The Gemara answers that she indeed may eat even a Shi'ur, but she must make sure not to eat a Shi'ur of food within Kedei Achilas Peras. Why, though, must she avoid eating within Kedei Achilas Peras, if her life is in danger?
The ROSH (Yoma 82a) answers that if her physical condition requires that she eat a lot of food within Kedei Achilas Peras on Yom Kippur, she certainly may eat what she needs, because Piku'ach Nefesh overrides Yom Kippur. The Gemara means that a pregnant woman is generally not in such a condition that she must eat so much and so fast. It generally suffices for her to eat a k'Zayis not bi'Chdei Achilas Peras.
The SEFER HA'CHINUCH (#313) implies that the Gemara refers to a case in which the pregnant woman is not in absolute danger. Rather, she merely is very weak, and thus she may eat a Shi'ur of food but only by pausing so that she not eat a Shi'ur within Kedei Achilas Peras. (Mordechai Zvi Dicker)
3) MOTHER'S MILK NEEDS NO "HECHSHER"
QUESTION: Rava states that the milk of a nursing woman who is a "Maga Tamei Mes" (she touched someone who was Tamei with Tum'as Mes) does not need Hechsher in order to become Tamei. Accordingly, a baby who drinks a Revi'is of milk from such a woman should become Tamei, just as anyone becomes Tamei when he eats or drinks a Shi'ur of Tamei food. Rava infers that the milk does not need Hechsher from a Mishnah that says that the milk of a woman, even the milk that comes out unintentionally, can become Tamei, even though it had no Hechsher (see RASHI, DH she'Lo).
However, it is not clear why a mother's milk does not need Hechsher. Why is milk different from all other foods?
(a) RASHI (DH v'Od) writes that the milk of a woman who is a Maga Tamei Mes is Metamei like the milk of a Nidah. The Torah teaches that the milk of a Nidah is Metamei like her limbs are Metamei. This is called the Tum'ah of a "Ma'ayan," which becomes Tamei without Hechsher.
Rashi (DH k'Shi'ur) agrees that there is no other Tum'as Ma'ayan other than the Tum'ah of a Nidah or Zavah. A Maga Tamei Mes does not have Tum'as Ma'ayan. Besides the fact that the Torah does not teach that Tum'as Ma'ayan applies to a Maga Tamei Mes, the law is that a Maga Tamei Mes who touches another person does not make the other person Tamei. Consequently, any fluid that comes from her certainly should not be Metamei another person. What, then, does Rashi mean when he compares the Tum'ah of the milk of a Maga Tamei Mes to the Tum'as Ma'ayan of a Nidah?
The Acharonim (see ARUCH LA'NER, MAYIM KEDOSHIM) explain that Rava is teaching that the milk of a woman is not viewed as something separate from her body, but rather it is viewed as a part of her body (as Rashi mentions, "as one of her limbs"). Since it is part of her, it has an inherent Tum'ah, and one who drinks it drinks something that is Tamei, and thus he becomes Tamei. Hechsher is necessary only when something must first attain the status of a "food" ("Ochel") in order to become Tamei. A liquid that is inherently Tamei needs no Hechsher. It is this point that the Ma'ayan of a Nidah proves. If her milk would not be considered part of her, then it would not be possible to consider it a Ma'ayan (like saliva or urine); it would need to emerge from the body, attain a status of food with Hechsher, and, after touching the outside of the Nidah's body, it would be Metamei only like an object that touched a Nidah (and not like a Nidah herself). From the fact that the Torah teaches that milk is Metamei like the Nidah herself, it is evident that the milk is a liquid that has the inherent Tum'ah of the woman from whom it came, and it does not need Hechsher in order to be Metamei the one who drinks it. Thus, while it is true that the milk does not have the status of Ma'ayanos ha'Nidah (as we asked), nevertheless it is Metamei the one who drinks it since it has an inherent Tum'ah.
(b) TOSFOS (according to the CHAZON ISH, beginning of Machshirin) understands that mid'Oraisa the milk of a Maga Tamei Mes needs Hechsher. However, since the milk of a Nidah is Metamei without Hechsher because of the Gezeiras ha'Kasuv of Ma'ayan, the Rabanan decreed that the milk of a Maga Tamei Mes is also Metamei without Hechsher, in order to prevent confusing the two. (Mordechai Zvi Dicker)
4) ONE WHO DRINKS ANY AMOUNT OF AN INTOXICATING BEVERAGE AND ENTERS THE MIKDASH
QUESTION: The Beraisa quotes the verse, "Yayin v'Shechar Al Tesht" -- "Do not drink wine or any other intoxicant" (Vayikra 10:9), which commands the Kohen not to enter the Beis ha'Mikdash while under the influence of an intoxicating beverage. The Beraisa asks that perhaps one is liable for entering the Mikdash after he drinks any amount of wine. The Beraisa explains that the word "Yayin" forbids one from entering the Mikdash after drinking any amount, but he is not Chayav for doing so.
The words of the Beraisa are difficult to understand. Why would one have thought that the Kohen would be Chayav for entering the Mikdash after drinking the smallest amount of wine? "Drinking" is always defined as having a Shi'ur of a Revi'is. Why should "drinking" wine and entering the Mikdash is different?
Moreover, why would one have thought that drinking any amount is not forbidden with an Azharah? Rebbi Yochanan in Yoma (73b) teaches that "Chatzi Shi'ur Asur Min ha'Torah" -- even a partial Shi'ur of an Isur is forbidden mid'Oraisa (but one is not punished for it).
ANSWER: When the Torah says, "Do not drink wine...," it does not mean that drinking wine is forbidden. There is nothing inherently wrong with drinking wine. Rather, the Torah prohibits one from entering the Beis ha'Mikdash while one is in a certain state (intoxication). The rule that "drinking" is always defined as having a Shi'ur of a Revi'is applies only when the act of drinking is itself Asur, but not when the act of drinking is merely a condition of the Isur (such as in this case, where the Isur is entering the Beis ha'Mikdash in a state of intoxication). This answers the first question.
Once we derive from a verse that the Torah forbids only wine that has the ability to intoxicate, we can understand the Beraisa's question that perhaps one is permitted to walk into the Beis ha'Mikdash, even l'Chatchilah, after drinking the smallest amount of wine. This is because the principle that "Chatzi Shi'ur Asur Min ha'Torah" applies only where the first half of the item is Asur, but it needs more of the same in order to make the person Chayav. In the case of the Isur of entering the Beis ha'Mikdash intoxicated, one who drinks less than a Shi'ur of wine might have no Isur whatsoever; a partial Shi'ur is not Asur at all, because it is not the drinking of the wine that is Asur (but rather entering the Beis ha'Mikdash while intoxicated). Hence, the verse teaches that even after drinking a drop of wine, one is prohibited l'Chatchilah from walking into the Beis ha'Mikdash. (Mordechai Zvi Dicker)