1) PLASTERING ONE'S BOR
OPINIONS: The Mishnah states that a person may not dig a Bor (pit) next to his neighbor's Bor unless he distances his own Bor at least three Tefachim from the wall of his neighbor's Bor, "and plasters it with plaster." The Gemara asks whether the Mishnah means that he must distance himself by at least three Tefachim and plaster his Bor with plaster, or that he may either distance his Bor or plaster it. The Gemara cites two proofs but does not answer its question conclusively.
What is the Halachah? Does a person who wants to build a Bor near his neighbor's Bor have to distance his Bor by three Tefachim and plaster it, or is he required to do only one of these two things?
(a) The RAMBAM (Hilchos Shechenim 9:1) writes that one must do both -- distance his Bor from his neighbor's Bor by three Tefachim and plaster his Bor with plaster.
There are two different explanations given for the ruling of the Rambam.
1. The MAGID MISHNEH explains that since the Gemara states that "it is obvious that the Mishnah means [that the person building the Bor must distance his Bor] 'and plaster it'," the Rambam understands that this is the Gemara's conclusion. Although the Gemara proceeds to refute its initial statement, this seems to be the Halachah because the Gemara calls it the obvious meaning of the Mishnah. This is also the way the RITVA explains the ruling of the Rambam. He adds that when the Gemara refutes its assumption that the Mishnah requires both conditions, the Gemara merely is suggesting possible refutations ("Dilma"), but it does not refute the assumption conclusively.
This is also the view of RABEINU YONAH, who writes that when the Gemara refutes this suggestion with a statement of "Dilma" ("perhaps..."), it merely is suggesting a possible way to refute the assumption, but it does not reject it conclusively. This also seems to be the view of the RIF, who quotes the wording of the Mishnah ("v'Sad b'Sid") but does not mention the Gemara's question.
2. The VILNA GA'ON (in BI'UR HA'GRA CM 155:51) explains that the Rambam's ruling is in accordance with his own view regarding unresolved questions in the Gemara. The Rambam maintains that in any case of a Safek in the Gemara, Beis Din must rule stringently, even if the Safek is a question that the Gemara asks but does not resolve ("Ba'aya d'Lo Ifshita"). The question of whether a Bor must be both plastered and kept at a distance is not resolved in the Gemara. Thus, a person who wishes to dig a Bor must both distance it by three Tefachim from his neighbor's Bor and cover it with plaster, since the Halachah may require both.
(However, the Bi'ur ha'Gra earlier (CM 155:26) explains the Shulchan Aruch (who rules like the Rambam) as the Magid Mishneh explains the Rambam, that the Gemara's refutation of its initial assumption is only a suggestion and not conclusive. See MA'AREI MEKOMOS of RAV CHANOCH KARELENSTEIN zt'l who discusses this apparent contradiction in the words of the Vilna Ga'on.)
(b) The ROSH writes that since this question is unresolved, Beis Din cannot force the builder of the Bor both to distance his Bor and to plaster it. Rather, it suffices for him to do one of the two things. This is also the ruling of the MORDECHAI (end of Bava Metzia) as cited by the BI'UR HA'GRA (CM 155:51).
The Vilna Ga'on writes further that the Rosh's ruling is in accordance with his own general position: that in any case (like this one) of an unresolved question in the Gemara, Beis Din should rule leniently.
Accordingly, the Rambam and Rosh disagree about how Beis Din rules with regard to a question in Halachah that the Gemara does not resolve. The Vilna Ga'on (Bi'ur ha'Gra CM 155:8, in "Likut") explains their reasons. The Rambam maintains that since mid'Oraisa one is prohibited to damage someone else's property, Beis Din must rule stringently in a case in which the Halachah is in doubt. (If, however, a person already built a Bor at too small a distance from his neighbor's Bor, Beis Din does not obligate him to remove his Bor, because, as the Shulchan Aruch there writes, "ha'Motzi me'Chaveiro Alav ha'Re'ayah.") The Rosh, on the other hand, maintains that even though the prohibition to cause damage to another person's property is mid'Oraisa, Beis Din rules leniently in this case because it is a monetary matter, and Beis Din adopts the lenient ruling in all questions of monetary law that the Gemara does not resolve. The Rambam maintains that Beis Din rules leniently in monetary matters only with regard to extracting money from someone who is "Muchzak," due to the principle of "ha'Motzi me'Chaveiro Alav ha'Re'ayah."
2) WHEAT SEEDS PLANTED AT THE SIDES OF A VINE SHOOT
QUESTION: The Gemara says that when one replants a shoot of a grapevine, he is not permitted to sow wheat seeds directly above the replanted vine shoots because of the Isur of Kil'ayim. However, one may sow wheat seeds on the sides.
Why is he permitted to sow wheat seeds on the sides of the vine? Even though the roots from the seeds will not extend to the sides (where the vine shoot grows), the roots of the vine shoot themselves will extend toward the seeds, and thus there should still be a problem of Kil'ayim. (RAMBAN, RABEINU YONAH, RASHBA, CHIDUSHEI HA'RAN, RITVA (19a), TOSFOS RID)
(a) The RAMBAN, RAN, and RITVA answer that it is not considered "Harkavah" (grafting) when the seeds and roots of wheat do not mix with the grapevine itself. When the seeds of wheat mix with the roots of the grapevine, even though the roots intermingle, this is not considered Kil'ayim. Only when the roots of the wheat mix with the grapevine itself is it considered Kil'ayim.
The Ramban adds that the reason why it is not considered Kil'ayim when the roots of one plant mingle only with the roots of the other (and not with the plant itself) is that in such a case, the roots of each plant absorb nourishment from the ground and produce a fruit that is not grafted. (The Ramban writes, however, that this reasoning needs further clarification.)
(b) RABEINU YONAH and the RASHBA answer that the roots of a grapevine descend further than three Tefachim into the ground (and they do not ascend upwards at all), and thus the roots that grow from the wheat seeds do not mingle with the roots of the grapevine. This is also the explanation of the RAN on the Rif in Kidushin (end of the first Perek).