1) HALACHAH: "TUM'AH" OF GRAVES OF NOCHRIM
OPINIONS: The Gemara cites a dispute among Tana'im about whether or not the graves of Nochrim are Metamei with Tum'as Ohel. Rebbi Shimon maintains that they are not Metamei b'Ohel, and the Chachamim maintain that they are. The Gemara concludes that even if they are not Metamei b'Ohel (as Rebbi Shimon says), they nevertheless are Metamei with Tum'as Maga u'Masa (by touching or carrying).
The Gemara's discussion has practical Halachic relevance for Kohanim today, since Kohanim are prohibited from becoming Tamei with Tum'as Mes. May Kohanim walk over graves of Nochrim or enter a cemetery of Nochrim?
(a) The RAMBAM (Hilchos Tum'as Mes 1:13) rules like Rebbi Shimon that graves of Nochrim are not Metamei b'Ohel. (The Rambam in TESHUVOS PE'ER HA'DOR #57 implies that even according to the Chachamim graves of Nochrim are not Metamei b'Ohel but only b'Maga u'Masa. According to Rebbi Shimon, they are not Metamei even b'Maga u'Masa. This also seems to be the view of the SEFER YERE'IM #322. According to this understanding of the Gemara, the Rambam rules like the Chachamim when he says that graves of Nochrim are not Metamei b'Ohel but only b'Maga u'Masa.)
However, TOSFOS (DH mi'Maga), the ROSH (Teshuvos 30:1), and other Rishonim here understand the Gemara in its straightforward sense and rule like the Chachamim that graves of Nochrim are Metamei b'Ohel. For this reason, the SHULCHAN ARUCH and REMA (YD 372:2) write that a Kohen nowadays should be stringent and not walk over graves of Nochrim. (TESHUVOS V'SHAV HA'KOHEN #75 is even more stringent and rules that a Kohen is forbidden, according to the letter of the law, from walking over graves of Nochrim, and it is not merely a stringency, since the majority of Rishonim rule that graves of Nochrim are Metamei b'Ohel.)
(b) Another issue affects the practical applications of this Halachah. The RA'AVAD (Hilchos Nezirus 5:16) rules that if a Kohen or Nazir is already Tamei with Tum'as Mes, he is not liable for touching another corpse. (The ROSH (Hilchos Tum'as Kohanim #6) cites RABEINU TAM who rules similarly that if a Kohen touches a corpse on a certain day, and on the same day he touches another corpse, he is not punished for touching the second corpse since that act does not extend the number of days which he must remain Tamei before Haza'ah.) The CHASAM SOFER (YD 339) points out that this appears to be the opinion of RASHI in Nazir as well. The RA'AVAD concludes that since all Kohanim today are already Tamei with Tum'as Mes, they are not liable for becoming Tamei with Tum'as Mes again.
Although the Rambam and Tosfos disagree and maintain that a Kohen who is already Tamei is still obligated to observe the prohibition against becoming Tamei, the MISHNEH L'MELECH (Hilchos Avel 3:5) suggests that a Kohen should be permitted to walk over graves of Nochrim because of a Sfek Sfeika. The first Safek is that some Poskim rule that a Nochri is not Metamei b'Ohel. The second Safek is that even if a Nochri is Metamei b'Ohel, some Poskim rule that a Kohen is not obligated today to avoid contact with Tum'ah. The DAGUL MEREVAVAH (YD 372) cites this view as the Halachah. However, in a note added later, the Dagul Merevavah retracts this opinion and says that even the Ra'avad does not permit a Kohen who is Tamei to touch a corpse. Rather, he merely rules that there is no punishment (Malkus) if a Kohen does so. The Chasam Sofer (ibid.), however, disagrees with the Dagul Merevavah and says that, at worst, the prohibition according to the Ra'avad is an Isur d'Rabanan, and therefore there is a Sfek Sfeika which permits a Kohen to walk into a cemetery of Nochrim.
(c) The VILNA GA'ON (in ADERES ELIYAHU, Parshas Chukas) as cited by the OR SAME'ACH (Hilchos Tum'as Mes 1:13) presents a novel ruling. The Vilna Ga'on asserts that even if the grave of a Nochri is not Metamei b'Ohel, it is Metamei b'Maga whether one touches it directly or whether one merely touches the gravestone or any object that rests directly above the body. The reason for this is as follows.
According to the laws of Tum'as Ohel, when there is less than a Tefach between the object of Tum'ah and the covering above it, the Tum'ah penetrates through the covering and rises. This Halachah is called "Tum'ah Retzutzah." The Vilna Ga'on rules that touching the covering through which the Tum'ah penetrates is the same as touching the source of the Tum'ah itself (the corpse). Similarly, walking over (and touching) the grave of a Nochri is like touching the corpse itself (unless, of course, there is a Tefach of space between the top of the corpse and the roof of the coffin or object above the corpse). Accordingly, a Kohen is forbidden from touching the grave of a Nochri.
The OR SAME'ACH points out that this approach resolves a number of problematic statements in the Gemara. One such statement is that of the Yerushalmi (cited by TOSFOS to 12a, DH she'Iber). The Yerushalmi relates that it happened once that all of the Kohanim in the Beis ha'Mikdash became Tamei when the skull of a Nochri (Aravnah ha'Yevusi, the previous owner of the land on which the Mizbe'ach was erected) was unearthed beneath the Mizbe'ach. The MISHNEH L'MELECH (Hilchos Beis ha'Bechirah 1:13) asks that according to those who rule like Rebbi Shimon, the skull should not have made anyone Tamei because no one touched it and it is not Metamei b'Ohel. According to the Vilna Ga'on's approach, the reason why the skull was Metamei the Kohanim is clear. The Kohanim touched the Mizbe'ach which rested on top of the Nochri's skull, and thus they became Tamei with Tum'as Maga.
Based on the view of the Vilna Ga'on, the Or Same'ach concludes that a G-d-fearing Kohen should be careful not to touch even the top of the grave of a Nochri. (The Rogatchover Ga'on, in TESHUVOS TZAFNAS PANE'ACH (Warsaw) #256, also discusses this issue and concludes that "Tum'ah Retzutzah" does not apply to the grave of a Nochri.)
2) THE MITZVAH OF PIRYAH V'RIVYAH
QUESTION: In the Mishnah, Beis Shamai and Beis Hillel disagree about the fulfillment of the Mitzvah of Piryah v'Rivyah. Beis Shamai maintains that one fulfills the Mitzvah only when he has two sons. Beis Hillel maintains that one fulfills the Mitzvah when he has one son and one daughter (see Chart). Beis Hillel derives his view from Creation, when Hash-m created the world with one male and one female, Adam and Chavah. Beis Shamai argues that "we may not derive that which is possible from that which is not possible" ("Ein Danin Efshar mi'she'Iy Efshar"). The world had to be created with one man and one woman (and not with two men) in order for mankind to propagate. The world could not have been created in any other way.
The Gemara implies that Beis Hillel maintains that we may derive "that which is possible from that which is not possible." However, the Gemara earlier (46a) states that the Halachah follows the view of Rebbi Akiva who rules that we may not derive Halachos in such a manner. It is unreasonable to suggest that Rebbi Akiva argues with Beis Hillel in this regard. (MAHARSHA)
(a) The MAHARSHA answers that the number of children one needs in order to fulfill the Mitzvah of Piryah v'Rivyah may be derived in such a manner, "Efshar mi'she'Iy Efshar," because there is a logical basis to say that one fulfills the Mitzvah with one son and one daughter. Such propagation provides replacement for the past generation and enables the world to continue to be populated.
(b) The ARUCH LA'NER writes that when no other source teaches the details of a Halachah, those details may be derived "Efshar" from "Iy Efshar." In the case of Piryah v'Rivyah, according to Beis Hillel there is no other source from which to derive the details, because Beis Hillel maintains that the details cannot be derived from Moshe Rabeinu as the Gemara describes. (See also TOSFOS CHAD MI'KAMAI.)
(c) Perhaps deriving the number of children needed to fulfill the Mitzvah of Piryah v'Rivyah from the creation of Adam and Chavah is not considered "Efshar" from "Iy Efshar." This is because it was possible for Hash-m to create two males. Had the Mitzvah of Piryah v'Rivyah required that one have two sons, Hash-m would have created the world with two males and one female in order to teach that one must have two sons in order to fulfill the Mitzvah. (He would have created a female only for the sake of propagating the world.) Since Hash-m did not create the world in that manner, but rather He created one male and one female, it must be that one fulfills the Mitzvah of Piryah v'Rivyah with one son and one daughter.
Beis Shamai -- who asserts that deriving the Halachah from Creation is considered deriving "Efshar" from "Iy Efshar" -- maintains that Hash-m created the world with the number of souls which a person must bear in order to fulfill the Mitzvah of Piryah v'Rivyah. Hash-m, therefore, created the world with only two persons. He could not have created two males, since the world could not have been populated from only two males. Beis Hillel maintains that since Hash-m did not create another male at Creation, we may derive from there that the Mitzvah of Piryah v'Rivyah is fulfilled with a son and a daughter. (M. Kornfeld)