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Can one make a neder or shevuah to perform a mitzvah (מצוה)? The Gemara (גמרא) on Nedarim 8a questions whether such a commitment violates "ein shevuah chal al shevuah" — you cannot layer a new oath atop a preexisting Divine obligation. The shiur explores the Ran's chakira between neder (object-based consecration) and shevuah (personal obligation), and unfolds the Rosh's novel conception that a neder of mitzvah is valid because it is a binding personal word, not an isur cheftza.
This shiur is an intensive analysis of Nedarim 8a, examining the Gemara (גמרא)'s question of ein shevuah chal al shevuah (one oath cannot take effect upon another oath) in the context of vows to perform mitzvos. The central issue is whether a person can make a neder or shevuah to perform a mitzvah (מצוה) when he is already Divinely obligated from Sinai (mushba v'omed me-Har Sinai). Rabbi Zweig opens by clarifying two distinct problems that might invalidate such a vow. First is the issue of lehatah or le-heitiv: a shevuah bitui requires an element of choice—something the person could do or refrain from doing. If one is already obligated in a mitzvah, there is no real choice, and the oath should not take effect. Second is the problem of ein shevuah chal al shevuah: even if there is theoretical choice (e.g., the person could exempt himself temporarily), the fulfillment of the neder or shevuah would simultaneously be the fulfillment of the mitzvah obligation. This creates a situation where the personal vow is layered upon the Divine "vow" from Sinai, which is forbidden.
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Nedarim 8a
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Why does the Gemara present a kal v'chomer for chuppah making kiddushin based on kesef not being maichel b'terumah, only to reject it? The shiur reveals that even the rejected kal v'chomer establishes a fundamental yesod: kiddushin is a kinyan for ishus (marital relationship), not merely a kinyan in hilchos kinyanim. The second kal v'chomer—from chuppah being gomeres—raises Tosafos's famous question.
Is the kesef used in kiddushin a true kinyan (acquisition), or merely a symbolic means of establishing marriage? The shiur analyzes multiple interpretations of the Gemara's derivation from ki yikach ki'ach mi'Sayeffron, exploring whether marriage constitutes ownership, and develops three fundamental approaches: (1) The Gemara proves ishus is a kinyan, (2) ishus is not a kinyan but uses kinyan-like means, or (3) ishus is a kinyan but the kesef may not be a ma'aseh kinyan. The Rambam's position—calling kesef "midivrei sofrim"—is explained as distinguishing between cross-referencing concepts versus borrowing terminology.
Why does the Mishna alternate between the language of "kinyan" (acquisition) and "kiddushin" (sanctification) when describing marriage? The shiur develops a fundamental distinction: marriage has two separate aspects—a kinyan dimension (ownership) and a kedusha dimension (sanctified relationship). Either aspect can initiate the bond, and understanding this dual nature resolves multiple textual and halachic questions throughout the sugya.
Does kiddushei shetar require a shevah pruta? The Ran presents two approaches to the Rif's position. The shiur develops the Ran's second pshat that shetar kiddushin functions as a hachnah (exchange) rather than a mere kinyan nesina. This reading has profound implications for isuri hana'ah, mechubar, and the relationship between kesef and shetar.
