1) CARRYING ON EREV SHABBOS CLOSE TO SHABBOS
QUESTION: The Mishnah teaches that a tailor may not carry his needle outside on Erev Shabbos close to Shabbos, lest he forget and carry it on Shabbos. What exactly is it that he might forget?
(a) The simple explanation is that he might forget that he is carrying the needle, and when Shabbos arrives he will inadvertently carry it outside.
(b) However, according to Rava (11b), the Mishnah cannot mean that we are concerned that the tailor will forget that he is carrying a needle. Rava maintains that the Rabanan do not make decrees to enforce pre-existing decrees (Gezeirah l'Gezeirah). Inadvertently carrying an object outside involves only an Isur d'Rabanan, and not an Isur d'Oraisa. An act of "Mis'asek" -- performing a forbidden act on Shabbos without awareness that one is performing that act -- is prohibited only mid'Rabanan. (This is in contrast to an act of "Shogeg," where a person is fully aware of what he is doing but he forgot the Halachah.) Accordingly, the Rabanan would not forbid one from carrying a needle on Erev Shabbos in order to prevent him from inadvertently carrying it outside on Shabbos, because such a decree would be a decree enforcing another decree.
Rather, TOSFOS (DH Shema) explains that the concern is that he will forget before Shabbos that he is carrying the needle, and then on Shabbos he will remember that he is carrying the needle, but at that time he might forget that it is Shabbos and inadvertently carry the needle outside. (Although one who forgets that it is Shabbos will likely transgress many additional (and more severe) prohibitions than merely carrying a needle into Reshus ha'Rabim, the Mishnah specifically teaches that one may not carry a needle before Shabbos because we assume that he will remember that it is Shabbos before he does a more obvious Melachah than carrying, and, furthermore, the act of carrying on Shabbos is preventable with such a decree.)
2) MAKING A DECREE TO ENFORCE ANOTHER DECREE
QUESTION: The Gemara explains that Abaye maintains that the Rabanan indeed make decrees to enforce other decrees. TOSFOS (DH Amar Abaye) points out that Abaye himself says elsewhere that the Rabanan do not make decrees to enforce other decrees. Tosfos explains that Abaye maintains that the Rabanan make decrees to enforce other decrees only with regard to the Melachah of Hotza'ah on Shabbos.
Why is Hotza'ah different from all other Isurim, for which the Rabanan do not make decrees to enforce their decrees?
(a) The RAN explains that since Hotza'ah is not an obvious act of Melachah (it is a "Melachah Geru'ah"; see Insights to Shabbos 2:1), it needs an additional safeguard.
(b) The TOSFOS HA'ROSH answers that since carrying items from one place to another is a common act and is done very frequently, the Rabanan deemed it necessary to make extra decrees to ensure that the prohibition against carrying is observed properly.
(c) The RAMBAN explains that whenever there is a strong similarity between a situation in which a particular act is Asur mid'Rabanan and a situation in which the same act is Asur mid'Oraisa (and there is a concern that people might confuse the two cases), when the Rabanan enacted their initial decree as a safeguard in the situation of the Isur d'Oraisa, they simply applied that same decree to the situation of the Isur d'Rabanan. In the case of the Gemara here, in which Abaye says that the Rabanan made a decree to prohibit one from standing in a Karmelis and drinking in a Reshus ha'Yachid, since a Karmelis is so similar to Reshus ha'Rabim, the Rabanan included a Karmelis in the decrees that they made with regard to Reshus ha'Rabim. This is also the approach of the MAHARAM and PNEI YEHOSHUA.
(This explanation is problematic, because the Gemara continues to cite proofs against Rava from Beraisos that discuss carrying an object into Reshus ha'Rabim in a manner that is forbidden only mid'Rabanan (such as carrying something on one's lapel). Such acts do not involve a Karmelis. Why did the Rabanan make decrees to enforce other decrees even when the domain under discussion is not a Karmelis? The Ramban answers that in those cases as well there is logical justification for enforcing rabbinical decrees, because wherever carrying is forbidden on Shabbos (such as from Reshus ha'Yachid into Reshus ha'Rabim), carrying should also be forbidden on Erev Shabbos, lest people confuse the two cases. That is, carrying on Erev Shabbos is so similar to carrying on Shabbos that the decrees made with regard to carrying on Shabbos apply to carrying on Erev Shabbos as well.)
(d) RASHI (DH Mai Lav) seems to understand that Abaye maintains that the Rabanan make decrees to enforce their other decrees in all prohibitions of Shabbos.