1) WAITING TWENTY YEARS TO DETERMINE IF A BABY WILL LIVE
QUESTION: Rebbi Avahu teaches, "Whe n a person has the signs (Simanim) of a Seris or an Aylonis, and when a baby is born after eight months of gestation, we do not rely on its external signs to determine the person's status until he reaches the age of twenty." The Gemara asks that a baby born after its eighth month cannot survive until the age of twenty. Why, then, does Rebbi Avahu say that we wait until he reaches the age of twenty to determine his status? The Gemara explains that Rebbi Avahu refers to a baby born fully developed; his hair and nails are developed like those of a full-term newborn. (This answer is in accordance with the Girsa of Rashi. According to the Girsa of the Rif (see Bach), however, the Gemara answers that Rebbi Avahu refers to a baby whose hair and nails are not fully developed. This Girsa will be explained below.)
The Gemara then cites a dispute between Rebbi and the Rabanan. The Rabanan assert that any baby born in the eighth month is an eighth-month baby. (An eighth-month baby is assumed to have physical deficiencies which render it inviable.) Rebbi argues that only a baby born in the eighth month with signs of prematurity (its hair and nails are undeveloped) has the status of an eighth-month baby. If the baby's hair and nails are fully developed, even though it was born in the eighth month it is presumed to be a healthy, viable baby (it is considered a "seventh-month baby" which was born one month late.)
The Gemara's intent in citing the dispute between Rebbi and the Rabanan is to explain the statement of Rebbi Avahu about a baby born in the eighth month. How, though, does their dispute shed light on Rebbi Avahu's statement? It is still unclear why it is necessary to wait twenty years to determine whether the child is an eighth-month baby. If the child lives, he obviously is not an eighth-month baby. Why must we wait twenty years?
ANSWERS:
(a) RASHI explains the Sugya in the most straightforward sense: it is necessary to wait twenty years in order to determine that the person is fit to live and that he is not a Nefel, a stillborn. Before twenty years have passed, we must suspect that the child is an eighth-month baby even though he was born with fully-developed hair and nails. If he lives until the age of twenty, we assume that he was a seventh-month baby that was born a month late. (Although Raban Shimon ben Gamliel maintains that a baby which survives for thirty days is not a Nefel, apparently Rebbi Avahu argues and maintains that we must wait twenty years in order to verify that the child is not a Nefel.)
This approach is very difficult to understand, as the RITVA and other Rishonim ask. How can the status of a healthy, fully-grown, nineteen-year-old man be in doubt, such that he might not have the status of a "living person"? It seems unreasonable to apply the stringencies of an eighth-month baby to such a person (for example, Shabbos may not be desecrated to save his life, or he is Muktzah and may not be touched on Shabbos)! Moreover, how can Rebbi Avahu, an Amora, disagree with the Tana, Raban Shimon ben Gamliel, without any source to support his view?
Perhaps Rashi means that according to Rebbi Avahu, although a baby born in the eighth month which survives for thirty days certainly is not considered an eighth-month baby (and therefore is not Muktzah on Shabbos), there still exists a strong possibility that he is a Tereifah and that he will not survive until the age of twenty.
A Tereifah certainly is considered a living person and is treated like one (Shabbos may be desecrated in order to save his life). The fact that he is a Tereifah affects only specific Halachos (for example, if a Tereifah's ox killed another person, the Tereifah is exempt from the payment of Kofer; see Sanhedrin 78a).
Rebbi Avahu means that a child born in the eighth month is not considered to be free of defects (that is, not a Tereifah) before the age of twenty. Until he reaches the age of twenty there remains a doubt that perhaps he is a Tereifah. Certainly, however, he is not treated like "a stone" (as he is treated within thirty days after birth); the fact that he lived passed the first thirty days indicates that he is not a Nefel (with regard to the Halachos relevant to a Nefel, such as the exemption from mourning for him if he dies after thirty days). However, after thirty days, he is considered a Safek Tereifah until he reaches the age of twenty. (M. Kornfeld)
(This approach explains the words of Rashi when he writes that the mother of an eighth-month baby may feed her baby on Shabbos without touching it "because there is a danger for both of them." Why does Rashi add that she may feed the baby not only because of the danger to her health, but also because of the danger to the baby's health? An eighth-month baby is not considered a living person, and thus concern for the baby's health should not permit the desecration of Shabbos! Indeed, Rashi elsewhere (and Rashi on the Rif here) does not mention the danger to the baby at all (as the BACH here points out). RABEINU AVRAHAM MIN HA'HAR cites the comments of Rashi as they appear in our texts, and he rejects Rashi's explanation because of this question. Perhaps Rashi understands that the Beraisa which allows the mother to "bend over and feed" the baby refers to both an eighth-month baby which was born with the signs of an eighth-month baby, and one that was born fully developed in the eighth month. The latter is not a definite Nefel; there is a chance that he will survive and be a Tereifah, or that he will survive and not even be a Tereifah. Although the child is "Muktzah," one certainly may desecrate Shabbos to save his life because of the chance that the child will survive despite its premature birth.)
(b) The RAMBAN (in Milchamos) explains simply that when Rebbi Avahu says that "we do not... determine the person's status until he reaches the age of twenty," he does not refer to his statement about an eighth-month baby, but only to his earlier statement about the Simanim of a Seris and an Aylonis. When a baby is born after eight months, certainly we do not have to wait "until he is twenty years old" to determine that he is not going to live. Rebbi Avahu means merely that we do not rely on physical signs alone to determine whether a baby is an eighth-month baby.
The Ramban's explanation is consistent with his Girsa of the Gemara. According to the Ramban's text, Rebbi Avahu refers to a child born undeveloped, without completed hair and nails. Rebbi Avahu teaches that even though the child has signs of an eighth-month baby, when we do not know for certain that the child was in utero for eight months we may not assume that he will die.
(c) RABEINU CHANANEL and the RA'AVAD (cited by many Rishonim) explain that Rebbi Avahu does not discuss whether or not a particular eighth-month baby will survive in the first place. He discusses only the laws of Seris and Aylonis. Rebbi Avahu teaches that if a child was born in the eighth month, even though his Simanim are complete (his hair and nails are fully developed) he is weaker than an ordinary child and has Halachic similarities to a Seris. Due to the weakness of his constitution, he does not reach puberty until he reaches the age of twenty. Any pubic hairs that grow before that age are deemed to be mole hairs and do not indicate the onset of puberty; he reaches that state only when two hairs grow after the age of twenty.
(According to the way TOSFOS CHAD MI'KAMAI quotes the Ra'avad, the Ra'avad suggested a second, similar explanation which he ultimately rejected in favor of the first. According to his second explanation, the weakness of the eighth-month baby takes the place of Simanei Seris. He is considered a Seris if two pubic hairs do not grow before the age of twenty even though he displays no other signs of a Seris. (Normally, when there are neither pubic hairs nor Simanei Seris, the child is not considered a Seris until the age of thirty-five; see Chart.) The fact that he was born in the eighth month takes the place of Simanei Seris.)
(d) The BA'AL HA'ME'OR prefers the Girsa of the Rif (as cited by the BACH), according to which Rebbi Avahu discusses a case in which the newborn did not have fully-developed hair and nails. He explains (like Rabeinu Chananel) that Rebbi Avahu teaches the laws of a Seris and not the laws of who is considered a living person.
The Ba'al ha'Me'or suggests, however, that when Rebbi Avahu mentions an eighth-month baby, he does not refer at all to an actual baby born after eight months. Rather, he means that the signs of a Seris are the same as the signs of an eighth-month baby! An eighth-month baby is recognizable by his limp hair and incomplete fingernails. Rebbi Avahu teaches that although the Beraisa (80b) which lists the signs of a Seris does not mention underdeveloped fingernails, that nevertheless is one of the signs of a Seris. When the Beraisa (80b) lists limp, underdeveloped hair as one of the signs of a Seris, it means that underdeveloped hair and nails indicate a Seris.

80b----------------------------------------80b

2) "SIMANIM" OF A "SERIS"
QUESTION: The Beraisa lists the Simanim (signs) of a Seris and Aylonis. Rav Huna and Rebbi Yochanan disagree about what makes a person a Seris. Rav Huna maintains that only when a person has all of the signs does he have the status of a Seris. Rebbi Yochanan maintains that even one sign is enough to determine that a person is a Seris.
The Gemara continues and says that if the person has "two hairs in his beard," all agree that he must have all of the Simanei Seris to be considered a Seris.
The Gemara's statement is difficult to understand. One of the signs of a Seris is that he has no beard. If he has two hairs in his beard, by definition he is not a Seris and does not have all of the Simanim of a Seris.
Moreover, Rebbi Yochanan maintains that if he does not have two hairs in his beard, he needs only one Siman to be considered a Seris. Why does he need even one Siman to be considered a Seris? The lack of hairs in his beard is itself a Siman that he is a Seris, and one Siman suffices to make him a Seris according to Rebbi Yochanan. (TOSFOS DH d'Havi and DH Ki)
ANSWERS:
(a) TOSFOS explains that if the person's beard is comprised of only two hairs, it is not called a "beard." He is considered "beardless" and may still have all of the signs of a Seris.
When Rebbi Yochanan says that one who does not have two hairs in his beard needs only one Siman to be considered a Seris, he means that the absence of hairs in his beard itself suffices as a Siman to make him a Seris. He does not need any other Siman since he has no beard.
(b) According to other Rishonim (see RITVA and OR ZARU'A), the Gemara does not refer specifically to facial hairs in one's beard. Rather, it refers to two hairs anywhere on the body. (These Rishonim either did not have the word "Zakan" in their text of the Gemara (like the Girsa of the Vilna Ga'on), or they interpreted that word to mean the "Zakan ha'Tachton," the lower hairs.)
The VILNA GA'ON points out that this explanation answers the questions of Tosfos here. According to this explanation, the Gemara refers to hairs elsewhere on the body and not in one's beard. Rebbi Yochanan means to say that a person who has two hairs on his body must have all of the Simanim of a Seris to qualify as a Seris, including the complete lack of facial hair.
The second question is answered as well. In addition to not having two pubic hairs, the person must have at least one other sign of a Seris -- such as the lack of a beard -- in order to be deemed a Seris. The fact that he lacks pubic hair alone does not render him a Seris.
According to this understanding, even when a person has two pubic hairs he may be a Seris if he has all of the Simanei Seris. Why, then, does the Beraisa (80a, 80b) state that a twenty-year-old becomes a Seris with Simanim when he does not have hairs?
According to Rebbi Yochanan, the Beraisa may refer to a Seris who has only some of the Simanei Seris, in which case he is a Seris only if he does not have two hairs. If, however, he has hairs, he is not considered a Seris until he has all of the Simanim.
However, according to Rav Huna -- who maintains that when one lacks hairs he still needs all of the Simanim to be considered a Seris -- why does the Beraisa specify that a Seris does not have two hairs? What difference does it make whether he has hairs or not? Once he has all the Simanim, he should be a Seris even if he has two hairs!
Apparently, the Vilna Ga'on understands that according to Rav Huna, even the presence of all of the Simanei Seris does not suffice to render him a Seris if he has two pubic hairs. When the Gemara says that everyone agrees that "if he has two hairs, he must have all of the Simanim," it means that he must have at least all of the Simanim to be a Seris. However, all of the other Simanim do not actually render him a Seris, according to Rav Huna. (M. Kornfeld)