There are four differences between a vow and a sh'vuat bitui:
a) With regard to a sh'vuat bitui, one oath cannot take effect while another is already in effect,1 and with regard to vows, a vow can take effect while another is already in effect.
b) When one attempts to extend the scope of an oath taken previously, he is not liable,2 and with regard to vows, one is.
c) A sh'vuat bitui can take effect only with regard to actions that are left to one's choice,3 while vows take effect with regard to mitzvot as well as actions that are left to one's choice.
d) A sh'vuat bitui can take effect with regard to entity of substance and an entity that is not of substance, 4 while vows take effect only with regards to entities of substance.
What is meant by the statement that a vow can take effect while another is already in effect? If a person says: "I will be obligated to bring a sacrifice if I eat this loaf [of bread]," [repeats]: "I will be obligated to bring a sacrifice if I eat it," he is liable [to bring a sacrifice] for every oath that he takes. Similar laws apply in all analogous situations.
What is meant by the statement that one who extends the scope of a vow taken previously is liable? He heard his colleague take a vow and said: "And I am like you" immediately thereafter,5 he is forbidden [to partake of] the substance that his colleague deemed forbidden.6 If a third person heard the second person say: "And I am like you," and he also said: "I am like you," [he is also forbidden]. Even if there are one hundred and each one says: "And I am like you" immediately thereafter the statements of the previous one," they are all forbidden.
Similarly, when one says: "This meat is considered forbidden to me,' and even after several days7 says: "This bread is like this meat," [the prohibition] is extended to the bread and it becomes forbidden. If afterwards, he said: "And this honey is like this bread, and this wine is like this honey," even if he mentions 100 [substances], they are all forbidden.
[The following rules apply when a person's] father or teacher died on a particular day and he took a vow to fast that day8 and [actually] fasted.9 If after years past, he said: "Let this day10 be considered as the day on which my father - or my teacher - died," he is forbidden to eat on that day. For he attached this day [to his existing vow] and caused it to be forbidden as the day which is forbidden for him. Similar laws apply in all analogous situations.
What is meant by the statement that vows take effect with regard to mitzvot as well as actions that are left to one's choice? When a person says: "Matzah is forbidden to me on Pesach night," "Dwelling in a sukkah on that holiday is forbidden to me," or "I am forbidden to take hold of tefillin," they are forbidden to him. If he ate matzah, dwelled in a sukkah, or took tefillin, he is liable for lashes.11 Similar laws apply in all analogous situations. Needless to say, one who says: "I am obligated to bring a sacrifice if I eat matzah on Pesach night," is obligated to bring a sacrifice.12 Similar laws apply in all analogous situations.
Why do vows take effect with regard to mitzvot and oaths do not take effect with regard to mitzvot? Because when a person takes an oath he forbids himself from [partaking of] the entity mentioned in the oath.13 When, by contrast, one takes a vow, he causes the entity mentioned in the vow to be forbidden to him.14 Thus when a person takes an oath to nullify a mitzvah, he is placing a prohibition upon himself and he is already bound by an oath [to observe that mitzvah] from Mount Sinai, and one oath does not take effect if another is already in effect. When, by contrast, a person causes an entity to be forbidden through a vow, the prohibition involves the entity itself and that entity is not under oath from Mount Sinai.
When you contemplate [the wording of] the Torah, it appears that their interpretation matches the explanation which our Sages received according to the Oral Tradition. For with regard to a sh'vuat bitui, [Leviticus 5:4] states: "Whether he will do harm or do good," i.e., speaking about permitted activities as we explained,15 e.g., whether I will eat or drink today, whether I will fast, or the like. With regard to vows, by contrast, [Numbers 30:3] states: "He shall do everything uttered by his mouth," without differentiating between matters associated with mitzvot and those left to our own volition.
When a person takes a vow to fast on the Sabbath or a festival, he is obligated to fast16 for vows take effect even when they involve [the nullification of] a mitzvah as explained.17 Similarly, if a person takes an oath to fast every Sunday or every Tuesday throughout his life and a festival or the day preceding Yom Kippur18 falls on that day, he is obligated to fast. Needless to say, this applies with regard to Rosh Chodesh. If, however, Chanukah or Purim fall [on these days], his vow is superceded by [the celebrations of] these days. Since the prohibition against fasting on them is based on Rabbinic decree, reinforcement is necessary.19 Hence, his vow is superceded by the Rabbinic decree.
What is meant by the statement that vows take effect only with regards to entities of substance? If one says: "My speech is like a sacrifice for you,"20 he is not forbidden to speak to him, because speech is not an entity of substance. Similarly, if he tells him: "My speech is forbidden to you," it is not like his saying: "my produce is forbidden to you," or "My produce is like a sacrifice for you," in which instance, [the produce] would be forbidden.
Therefore, if a person tells a colleague: "[It is like a vow for] a sacrifice that I will not speak with you," "...that I will not act on your behalf," or "...that I will not go with you," or he told his wife, "[It is like a vow for] a sacrifice that I will not be intimate with you," his vow does not take effect in all these instances. For this is as if he is saying: "My speech, going, actions, or intimacy is like a sacrifice, and none [of these are] entities of substance.21
When, by contrast, a person says: "Let my mouth be forbidden to speak, my hands to act, my feet to walk, and my eyes to sleep," his vow is effective with regard to them.22 Therefore if a person tells a colleague: "My mouth is like a sacrifice with regard to speaking with you, my hands [are so] with regard to acting on your behalf, and my feet [are so] with regard to going with you," he becomes forbidden.
Similarly, one who tells a colleague: "I will be obligated to bring a sacrifice if I speak to so-and-so" or "...if I don't speak to so-and-so," he is obligated to bring a sacrifice if he violates this commitment. Similarly, if he took a vow in which he said: "[If] I spoke [to so-and-so, I must bring a sacrifice]" or "[If] I did not speak..." or the like, [he is liable]. For these are not vows in which he accepts prohibitions upon himself23 whose ground rules we are explaining here, but vows of dedication.24
Although when a person takes a vow concerning entities that are not of substance and forbids them, the vow does not take effect with regard to them, we do not rule that he should act as if they are permitted. [Instead,] since he willingly [took a vow] forbidding them to him, [according to Rabbinic decree]25 the vow took effect with regard to them. Although they are not forbidden, we give him an opportunity [to ask for the vow's release] from another vantage point and then release the vow, so that he will not act frivolously with regard to vows.26
This concept can be explained as follows: As stated in Halachah 6, an oath creates a prohibition on the person taking the oath (the gavra), i.e., the article is essentially permitted, he has accepted a prohibition on himself not to partake of it. Hence, it is not significant whether the article is of substance or not. With regard to vows, by contrast, the article itself (the cheftzah) becomes forbidden. Hence, for that prohibition to take effect, the article must be of substance (Radbaz). See Halachah 10 for an illustration of how this principle is effective with regard to vows.
I.e., he is extending the scope of his colleague's vow, so that it includes not only his colleague, but he himself.
Here also the concept can be explained according to the above difference. Since an oath involves an obligation on the person taking the oath (gavra), it cannot be extended to include another individual, for each person must take his own oath. With regard to a vow, by contrast, since the prohibition caused by the vow is associated with a substance (cheftzah), another person can also extend the prohibition to himself (Rabbenu Nissim).
In this instance, it is not necessary to make the statement immediately thereafter the first vow. Since the meat is visible before us, one can attach a vow to it. With regard to the previous halachah, by contrast, we are speaking about a subject that cannot be seen. Hence, unless the statements are made immediately after each other, there is no way we can be certain of the meaning of the statement: "And I am like you" (Radbaz).
For breaking his vow. The Rama (Yoreh De'ah 215:1) mentions a view that maintains that the person should be given corporal punishment for taking such a vow and should be compelled to ask to have the vow released.
I.e., the prohibition involves the cheftzah, the article. Once it is forbidden, it is forbidden to fulfill the mitzvah by partaking of it or using it, for a positive commandment does not supercede a negative commandment. It would be a mitzvah fulfilled through a transgression which is a forbidden act (the Rambam's Commentary to the Mishnah, loc. cit.).
Despite the fact that by doing so he negates the mitzvah of taking pleasure in the Sabbath and festivals. The Ra'avad clarifies that the matter is dependent on the wording he used in his oath. If on the Sabbath, he said: "I will fast today," he is forbidden to keep his vow.
In contrast to the Sabbath and festivals where the obligation to eat is of Scriptural origin. This is a general principle in Talmudic Law. There are times when our Sages gave their decrees greater power than Scriptural Law, for Scriptural Law is revered by the people at large and does not require reinforcement. If, by contrast, Rabbinic Law was abrogated in such instances, it might lead people to take leniencies even when uncalled for (see Ta'anis 17b; Kessef Mishneh). The Radbaz explains that if keeping one's vow was allowed to override a Rabbinic decree, then there would be no point in making such decrees. For people could nullify them by taking vows. For example, a person could take a vow to drink ordinary gentile wine.
The Kessef Mishneh also explains that although the obligation to eat on Rosh Chodesh and the day preceding Yom Kippur is also of Rabbinic origin, since our Sages found an allusion to it in the Torah, it is less likely that people will treat it lightly.
See Nedarim 15a. Similarly, in his Commentary to the Mishnah (Nedarim 2:1), the Rambam writes that the prohibition against desecrating one's word applies with regard to these vows. Compare to Chapter 4, Halachah 4.
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