Resources for Nedarim 13
The גמרא brings a מחלוקת תנאים as to whether you can be מתפּיס a נדר with a בכור. רבי יעקב says you can and רבי יהודה says you cant. תוספות explains that even רבי יעקב agrees that you can’t be מתפּיס בדבר האסור in general. He just holds that בכור is an exception because there is a מצוה להקדישו. There is a lot of discussion as to what that הקדשה that a person does to the בכור accomplishes. There are three different שיטות. The קובץ שיעורים in מסכת שבועות אות ז says that while it is a מצוה להקדישו it really doesn’t accomplish anything in terms of making it more הקדש. However, since the words have the power to make the animal הקדש in general, it is considered a דבר הנדור. He compares it to the fact that there is an איסור to be מטיל מום בקדשים and that איסור applies even if the animal is already a בעל מום. We see from there the concept that even though something is already in one status it is still meaningful when one does something that had the power to create that status on its own. A second שיטה is that of the מחנה אפרים. The מחנה אפרים in הלכות נדרים סימן י is discussing the ברייתא from the previous דף that says that if one is מתפּיס with the נדר one made to not eat on צום גדליה it is not considered a דבר האסור because צום גדליה is only an איסור דרבנן. רש"י in שבועות דף כ (which is where this is spelled out) says that this can’t be a correct גירסא because even if צום גדליה were an איסורי דאורייתא it would still be a good התפסה since נדרים are חל on existing איסורים. The רא"ש disagrees because even though it’s true that the נדר would be חל, when a person is מתפּיס he is being מתפּיס אעיקרא which is the original איסור so it would be מתפּיס בדבר האסור. The מחנה אפרים does not understand this because we see that בכור is considered a דבר הנדור according to ר׳ יעקב because there is a מצוה להקדישו. You see from there that a person is מתפּיס on the נדר portion and not the existing איסור. The אור שמח in הלכות נדרים פּרק ג׳ הל׳ ה answers this question by saying that by בכור your נדר is intended to give it the exact same קדושת בכור so in that case you have דעת for the נדור one. According to both the מחנה אפרים and the אור שמח, it seems they understood that when a person is מקדיש his בכור it’s a fully valid הקדשה/נדר and there are now two איסורים on this בכור –one from birth and one which you just created. There is a third שיטה of the מנחת שלמה here who says something even stronger than this. He says that not only is the הקדשה on the בכור impactful, it even replaces the original קדושה! In other words, once you are מקדיש it the קדושה מן השמים leaves and is replaced by your הקדשה! He says that it is משמע that way from theרמב"ם because theרמב"ם says in הלכות בכורות פּרק א׳ הל׳ ד the following: “מצוה להקדיש בכור בהמה טהורה וכו׳ ואם לא הקדישו הרי זה מתקדש מאליו”. It sounds from that that the main הקדשה comes from the owner. As an aside, he also mentions that the מרומי שדה says here that nowadays if one has a בכור בהמה טהורה one should not be מקדיש it since we can’t be מקריב it anyway and the הקדשה is a צורך הקרבה. רב שלמה זלמן ז"ל doesn’t know what the source for that is or why that should be true.
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