QUESTION: The Mishnah (17a) states that an oath of Nezirus takes effect on an oath of Nezirus, but a Shevu'ah does not take effect on a Shevu'ah. The Gemara cites Rav Huna who explains that this means that a person who makes, at one time, two oaths to become a Nazir on two consecutive days must observe thirty days of Nezirus for his first oath, and when he finishes the first Nezirus he must observe another thirty days of Nezirus for his second oath. In contrast, when one makes two Shevu'os, a Shevu'ah that he will not eat figs and then a Shevu'ah that he will not eat figs and dates together, the second Shevu'ah does not take effect since he is already prohibited from eating figs (by the first Shevu'ah).

Why does the Halachah in the case of Nezirus differ from the Halachah in the case of Shevu'ah? If a Shevu'ah cannot take effect on a pre-existing Shevu'ah because the first Shevu'ah already prohibits him from part of the items he mentioned in his second Shevu'ah, so, too, one's acceptance of a second Nezirus should not take effect because the first twenty-nine days of that Nezirus are already included in the first Nezirus! Although the Gemara (18a) cites a verse which teaches that Nezirus takes effect on Nezirus, "Nazir l'Hazir" (Bamidbar 6:2), from that Gezeiras ha'Kasuv it should be derived that a Shevu'ah takes effect on a Shevu'ah as well! The Gemara earlier (3a) teaches that the Halachos of Nedarim may be derived from Nezirus through a Hekesh, and the RAN (beginning of 7a) explains that all of the Halachos learned from Nezirus to Nedarim apply also to Shevu'os because of a Hekesh between Nedarim and Shevu'os (it is that Hekesh which teaches that Yados works for Shevu'os). Accordingly, the verse of "Nazir l'Hazir" should teach that a Shevu'ah takes effect on a Shevu'ah, just as it teaches that a Nezirus takes effect on a Nezirus.

ANSWER: A logical reason dictates that a second Shevu'ah should not take effect on an earlier Shevu'ah when the second Shevu'ah includes objects already prohibited by the first Shevu'ah. Since the second Shevu'ah cannot take effect on the objects which are already prohibited by the first Shevu'ah, and, if it takes effect, it can take effect only on the other objects, if the Shevu'ah were to take effect it would not be the same Shevu'ah which the person desired! Perhaps the person intended to make a Shevu'ah that includes all of the objects he mentioned, and he did not want to make a Shevu'ah on only part of the objects he mentioned. Since he might not have wanted to make a Shevu'ah on only part of the objects, the entire Shevu'ah does not take effect. (This logic is related to the concept of "Neder (Shevu'ah) she'Hutar Miktzaso, Hutar Kulo" (25b). Even though some Tana'im maintain that a Neder (or Shevu'ah) which was partially annulled does not become completely annulled, they apply that ruling only when the person mistakenly included in his Neder something which he had no deliberate intention to include. Since he did not want to include it in his Neder, the annulment of that part of his Neder does not affect the rest of his Neder. In contrast, when he wants to include two things in his Neder but the Torah does not let him include both things, everyone agrees that the principle "Neder she'Hutar Miktzaso Hutar Kulo" applies and the Neder does not take effect.)

Nezirus, however, is different. The Torah teaches that the second Nezirus which one accepted upon himself does take effect, and the laws of Shevu'ah cannot be derived from Nezirus in this regard. The reason why the second Nezirus takes effect is that although the first twenty-nine days of that Nezirus cannot take effect because he is already forbidden (from wine, haircuts, etc.) due to the first Nezirus, the thirtieth day of the second Nezirus can take effect, since the first Nezirus does not extend to that day. On that day, he will be obligated to observe one day of Nezirus. However, once he is obligated to observe that day of Nezirus, he becomes obligated to observe a full thirty days of Nezirus because of the rule that Nezirus cannot take effect for less than thirty days (Nazir 7a). The Nezirus of that day automatically expands into a thirty-day Nezirus.

Hence, even if the person intended to observe the second Nezirus for thirty days starting from the day after he made his oath (so that twenty-nine of those days overlap with the first Nezirus) and in practice his oath takes effect in a way in which he did not necessarily want (rather, he wanted a one-day Nezirus after the conclusion of his first thirty days of Nezirus), that one-day Nezirus expands into thirty days of Nezirus, making him a Nazir for thirty days as a result of his second oath of Nezirus. This is what the Torah means when it says that Nezirus takes effect upon Nezirus. In contrast, in the case of a second Shevu'ah, when the first half of the Shevu'ah does not take effect, the Shevu'ah which could take effect (on the other objects he listed) does not fit the words he articulated. Since there is a strong logical basis to distinguish between Shevu'ah and Nezirus, the laws of Shevu'ah cannot be learned from the laws of Nezirus in this regard, even through a Hekesh. (The principle of "Ein Meshivin Al ha'Hekesh" means that a Hekesh cannot be disproved merely on the basis of a Chumra or Kula which exists in one of the two categories being compared to each other (see Insights to Menachos 82:2). (This is in contrast to a Kal v'Chomer and a Binyan Av, which can be refuted based on a Chumra or Kula.) However, a strong logical argument certainly can refute a Hekesh. See Insights to Yevamos 3:2 in the name of the SEFER KERISUS).