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1)

(a) If someone who wants his place of residence (over Shabbos) to be in the street, and he then places his Eruv on a wall in the street which is within ten Tefachim - we have already learned that everyone has four Amos from the place where he is standing, which is called (with regard to the Din of Eruv) a Reshus ha'Yachid. That being the case, his Eruv would be effective whether he placed the Eruv above ten Tefachim or below it. Consequently, we are forced to establish the Beraisa which differentiates between an Eruv that is above ten Tefachim and one which is below that, by a wall which is more than four Amos away.

(b) If he placed his Eruv below ten Tefachim it is effective - due to the fact that both he and his Eruv are in the same Reshus (ha'Rabim), and because, during Bein Hashemashos, he is able to bring the Eruv to his 'residence' by carrying it less than four Amos at a time. When the Eruv is placed above ten Tefachim however, his Eruv is not valid, since he is in a Reshus ha'Rabim and his Eruv, in a Reshus ha'Yachid. Nor is there any Heter to carry from one Reshus to the other, even during Bein Hashemashos.

(a) If he intends his place of residence to be a dove-cot (with different levels) or in a large cupboard above ten Tefachim, and he places his Eruv above ten Tefachim (though not in his actual place of residence), the Eruv is effective, despite the fact that he is forbidden to take the Eruv from the one shelf in the cupboard to the other (via the air above the Reshus ha'Rabim) - because he is able to move down to where the Eruv is, and eat it there (which is nevertheless called 'Hu ve'Eruvo be'Makom Echad', since they are both in the Reshus ha'Rabim). However, if the Eruv is placed below ten Tefachim, it remains forbidden to take the Eruv to him (since it constitutes carrying from a Karmelis to a Reshus ha'Yachid via the air of a Reshus ha'Rabim). Nor will the fact that he is able to move down to the Eruv and eat it there, because, since he acquired his residence in a Reshus ha'Yachid, and he now wants to eat his Eruv in a Karmelis, that is still called 'Hu be'Makom Echad ve'Eruvo be'Makom Acher'.

(b) It is forbidden to move the Eruv from its location to his place of residence, even when both are above ten Tefachim - because it is forbidden to carry from a Reshus ha'Yachid to a Reshus ha'Yachid via the air of a Reshus ha'Rabim - even above ten Tefachim (where the air itself is a Reshus ha'Yachid.

(c) According to Rebbi Yirmiyah, asks the Gemara, the Eruv should be valid, even if it was placed below ten Tefachim, because of 'Ho'il' - meaning that one could have knocked the cupboard to the ground, in which case his place of residence would change to below ten Tefachim, and he would then be taking from a Karmelis to a Karmelis via a Reshus ha'Rabim, which is only de'Rabbanan, and this Tana holds like Rebbi (that Chazal did not decree on a Shevus, during Bein Hashemashos). Note: It is not clear however, how this differs from the actual case in question (i.e. even without Ho'il), since carrying from a Karmelis to a Reshus ha'Yachid via the Reshus ha'Rabim is also no more than an Isur de'Rabbanan (see Tosfos DH 'O').

(a) We are speaking here, answers the Gemara, about ...

1. ... a cupboard that is attached to the wall with nails, and cannot be moved from the wall, in which case, 'Ho'il' will not be applicable?
2. ... a long cupboard that is more than four Amos tall. Consequently, even if we knock it over, the new place of residence (where the top of the cupboard is now lying) will be at least four Amos away from where the Eruv is placed, in which case 'Hu be'Makom Echad ve'Eruvo be'Makom Acher' will apply, despite the fact that both locations are in the same Reshus.

(b) If the Eruv was attached to a string, which one could pull up from the one shelf to the other via a skylight in the cupboard - the Eruv would be valid even if it was below ten Tefachim, because then he would be pulling the Eruv from a Karmelis to a Reshus ha'Yachid directly, which involves no aspect of d'Oraysa at all.

4)

(a) If the pit is situated in a Reshus ha'Rabim - then it will depend upon where he intends to acquire his residence. If he intends it to be outside the pit, then his Eruv will not be valid, because he is in one Reshus, and his Eruv is in another (so why does our Mishnah validate it). Whereas if he intends to acquire his residence inside the pit, then it is obvious that his Eruv will be valid (in which case, the Mishnah is redundant).

(b) Our Mishnah must therefore be speaking - when the pit is in a Karmelis, and where he intends to acquire his residence outside the pit. The Chidush of our Mishnah - is that, even though taking the Eruv from the Reshus ha'Yachid to a Karmelis constitutes an Isur de'Rabbanan, the Eruv is nevertheless valid, because the Tana of our Mishnah holds like Rebbi, who maintains that Chazal did not decree Shevusin during the period of Bein Hashemashos, which is when the Eruv becomes operational.

(a) Since the Tana invalidates an Eruv on top of a reed or a stake if it is attached (although this involves nothing more than an Isur de'Rabbanan) he must hold like the Rabbanan of Rebbi - who decree Shevusin even during the Bein Hashemashos period. Consequently, it will be forbidden to remove the Eruv even during Bein Hashemashos, which will render the Eruv invalid.

(b) According to Ravina, the author of this Mishnah could even be Rebbi - who will agree by a reed or a stake, compared to a tree, are soft, and there is a strong suspicion that one may come to break a twig (see Ritva).

(a) When the town hosted numerous troops, and there was no room for the Talmidim in the Beis-Hamedrash - Rav Nachman advised them to go down to the fields and bend reeds to form stools to sit on.

(b) Rav Nachman was speaking about *soft* reeds (on which Chazal did not issue any decree), whereas our Mishnah is speaking about a reed that has hardened, and now resembles a tree (though it is somewhat softer than it).

(c) One Beraisa considers reeds and bushes as trees, which are *not* Kil'ayim in a vineyard, whereas another Beraisa considers them vegetables, which *are*. Clearly, the first Beraisa is referring to hardened reeds, and the second, to soft ones.

(d) The Mishnah which writes 'Ein Markivin Pigam al Gabei Kidah Levanah' - categorizes 'Kidah Levanah' as a tree; Whereas plain Kidan is a vegetable, and which therefore constitutes Kil'ayim when planted in a vineyard.

(a) The Gemara initially considers an Eruv placed in a cupboard whose key is lost, to be 'Hu be'Makom Echad, ve'Eruvo be'Makom Acher' - because his Eruv is inaccessible.

(b) The Gemara establishes our Mishnah by a cupboard made of un-cemented bricks - Such a cupboard is easy to take apart, and, according to Rebbi Meir, taking it apart is even permitted. Consequently, the Eruv *is* accessible.

(c) The Tana Kama of Rebbi Meir, who permits taking from the hole, if some of the bricks caved in, but not to actually remove any bricks - is Rebbi Shimon, who does not hold of Muktzah.

(d) Our Mishnah pertains to Yom-Tov, not to Shabbos - since Shmuel is quoted as saying that Rebbi Meir's Din is confined to Yom-Tov and does not extend to Shabbos.

(a) Rebbi Eliezer says that if the key of a tower made of loose bricks got lost, the Eruv is invalid.

(b) The third Tana holds that, if the key was lost in town, the Eruv is valid (because he holds like Rebbi Shimon, who maintains that the roofs and the enclosures of the town are all considered one - and that one may carry from one to the other). Consequently, if the key is found, he will be able to carry it to the cupboard and open it; whereas if it was lost in the fields, then even if it is found, he will have to leave it where it is.
Consequently, he will be unable to open the cupboard, and the Eruv will be invalid. This Tana however, is speaking about Shabbos, not about Yom-Tov, when it is permitted either way, though not for Rebbi Meir's reason, but because we contend with the key being found. Note: We are assuming that when he lost the key in town, he lost it in the vicinity of his home, and not in the street.

(c) It is conceivable, explains Rashi (in his second explanation, that Rebbi Eliezer in our Mishnah, who invalidates the Eruv, is speaking about when the key was lost *in the field*, and he can still be the author of the Beraisa, which permits the Eruv when the key was lost *in town*. (See however, Tosfos 35a DH 'Chasurei').

(d) The Tana Kama of this Beraisa holds like Rebbi Meir in our Mishnah, who validates the Eruv, irrespective of where the key was lost.