Halacha

הלכה א
מִי שֶׁנִּשְׁבַּע שֶׁזָּרַק אִישׁ פְּלוֹנִי צְרוֹר לַיָּם וְהוּא לֹא זָרַק. אוֹ שֶׁלֹּא זָרַק וְהוּא זָרַק. הֲרֵי זֶה חַיָּב בִּשְׁבוּעַת בִּטּוּי. וְאַף עַל פִּי שֶׁאֵינוֹ בִּלְהַבָּא שֶׁהֲרֵי אֵינוֹ יָכוֹל לְהִשָּׁבַע שֶׁיִּזְרֹק וְשֶׁלֹּא יִזְרֹק אִישׁ פְּלוֹנִי. וְכָל מִי שֶׁנִּשְׁבַּע עַל אֲחֵרִים שֶׁיַּעֲשׂוּ כָּךְ וְכָךְ אוֹ לֹא יַעֲשׂוּ אֲפִלּוּ הָיוּ בָּנָיו אוֹ אִשְׁתּוֹ אֵינוֹ חַיָּב בִּשְׁבוּעַת בִּטּוּי. שֶׁהֲרֵי אֵין בְּיָדוֹ לֹא לְקַיֵּם וְלֹא לְבַטֵּל. וּמַכִּין אוֹתוֹ מַכַּת מַרְדּוּת שֶׁהֲרֵי אֵין בְּיָדוֹ לְקַיֵּם שְׁבוּעָה זוֹ וְנִמְצָא גּוֹרֵם לִשְׁבוּעַת שָׁוְא:
כסף משנה
1.
When a person takes an oath that so-and-so threw a stone into the sea and he did not do so, or [he took an oath that] he did not throw it and he did, he is liable for taking a [false] sh'vuat bitui. [This applies] even though there is no [possibility of him taking such an oath] with regard to the future.1Although the concept of a sh'vuat bitui applies both with regard to the past and the future (Chapter 1, Halachah 2), it is not necessary that every sh'vuat bitui have both a past and a future component. For he cannot take an oath that so-and-so will throw [an article] or will not throw it.
[Indeed,] any person who takes an oath with regard to other people's [conduct - that they will or will not perform a particular activity is not liable for taking a [false] sh'vuat bitui. [This applies even if the person concerned] is his son or wife. For it is not within his potential to keep or nullify the oath. He is given stripes for rebellious conduct since it is not within his potential to keep this oath. Thus he is causing an oath to be taken in vain.2As the Rambam continues to explain, the oath is not necessarily false, because the other people may do what he postulated. Rashi (Sh'vuot 25a) considers this a false oath. The Siftei Cohen 236:4 quotes Rashi's view.

הלכה ב
וְלָמָּה אֵינוֹ לוֹקֶה מִשּׁוּם שְׁבוּעַת שָׁוְא שֶׁהֲרֵי אֶפְשָׁר לְאוֹתָן אֲחֵרִים שֶׁיִּשְׁמְעוּ מִמֶּנּוּ וְתִתְקַיֵּם שְׁבוּעָתוֹ. וְנִמְצָא כְּשֶׁמַּתְרִים בּוֹ בְּעֵת שֶׁנִּשְׁבַּע הַתְרָאַת סָפֵק שֶׁאֵין לוֹקִין עָלֶיהָ אֶלָּא אִם כֵּן הָיָה לָאו שֶׁבּוֹ מְפֹרָשׁ בַּתּוֹרָה כְּמוֹ שֶׁיִּתְבָּאֵר בְּהִלְכוֹת סַנְהֶדְרִין. וְאֵין אוֹתָן אֲחֵרִים נִזְקָקִין לְקַיֵּם דִּבְרֵי זֶה הַנִּשְׁבָּע אֶלָּא אִם כֵּן עָנוּ אָמֵן כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
2.
Why isn't he liable for lashes for taking an oath in vain? For it is possible for those other people to heed his [words] and keep his oath. Thus when he is given a warning at the time he takes the oath, the warning is of doubtful status. In such an instance, one is not given lashes because of it unless the prohibition is explicitly stated in the Torah, as will be explained in Hilchot Sanhedrin.3The Radbaz and the Kessef Mishneh point out several difficulties with the Rambam's words. Firstly, in Hilchot Sanhedrin, the Rambam does not make such statements explicitly. The only mention of a warning of a doubtful status is in Hilchot Sanhedrin 16:4. From those statements and those here, it appears that the Rambam considers such a warning as significant. There he does not explain the distinction of whether the prohibition is explicitly mentioned in the Torah or not. Also, the prohibition against taking a false oath is explicitly mentioned in the Torah. The Radbaz explains that the intent is that the concept that such an oath is considered as having been taken in vain is not explicit in the Torah and may not be known by an ordinary person. Other people are not bound to fulfill the words of the person who took the oath unless they responded Amen, as we explained.4As stated in Chapter 2, Halachah 1, whenever one responds Amen to a colleague's oath, he is bound by it.

הלכה ג
וְאִם קִיְּמוּ דְּבָרָיו הֲרֵי אֵלּוּ מְשֻׁבָּחִין שֶׁלֹּא הִרְגִּילוּ לְהוֹצִיא שְׁבוּעָה לַשָּׁוְא:
כסף משנה
3.
If they fulfilled his words,5The Rama (Yoreh De'ah 236:2) mentions two opinions. One emphasizes that the one who took the oath must certainly fulfill it. For example, if one takes an oath to marry a woman, the oath is considered as having been taken in vain, because the woman may not consent. Nevertheless, if she does consent, the man should keep his word and marry her. The other, however, does not consider this as an oath taken in vain, but rather as a false sh'vuat bitui. they are praiseworthy, for [in this manner,] they did not habituate [the person who took the oath] to take an oath in vain.6The Ma'aseh Rokeach maintains that even if the involved parties fulfill the oath, the person taking it is given stripes for rebellious conduct, for he should never have taken such an oath.

הלכה ד
בַּמֶּה דְּבָרִים אֲמוּרִים כְּשֶׁנִּשְׁבַּע עַל דָּבָר שֶׁאֵינוֹ בִּרְשׁוּתוֹ. כְּגוֹן שֶׁנִּשְׁבַּע רְאוּבֵן עַל שִׁמְעוֹן שֶׁלֹּא יֵלֵךְ בִּסְחוֹרָה אוֹ שֶׁלֹּא יֹאכַל בָּשָׂר וְכַיּוֹצֵא בָּזֶה. אֲבָל אִם נִשְׁבַּע רְאוּבֵן שֶׁלֹּא יִכָּנֵס שִׁמְעוֹן בְּבֵיתוֹ וְשֶׁלֹּא יֵהָנֶה מִנְּכָסָיו וְעָבַר שִׁמְעוֹן וְנִכְנַס לְבֵיתוֹ שֶׁל רְאוּבֵן וְנֶהֱנָה מִנְּכָסָיו שֶׁלֹּא מִדַּעַת רְאוּבֵן רְאוּבֵן פָּטוּר שֶׁהֲרֵי הוּא אָנוּס וְשִׁמְעוֹן חַיָּב שֶׁעָבַר עַל דָּבָר הָאָסוּר לוֹ לַעֲשׂוֹתוֹ. שֶׁלֹּא נִשְׁבַּע זֶה אֶלָּא עַל דָּבָר שֶׁהוּא בִּרְשׁוּתוֹ. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה:
כסף משנה
4.
When does the above apply? When he took an oath concerning a matter that was not in his domain. For example, Reuven took an oath that Shimon would not go on a commercial journey, not eat meat, or the like.7For in these instances, he has no control over the other person's actions. [Different laws apply,] however, should Reuven take an oath that Shimon may not enter his home and may not derive any benefit from his property. If Shimon transgressed and entered Reuven's house and benefited from his property without Reuven's knowledge, Reuven is exempt, for [his oath was violated] due to forces beyond his control.8For he did not know of Shimon's actions. Shimon is liable, for he performed a deed prohibited to him. For Reuven took an oath only with regard to a matter within his property.9The Tur questions the Rambam's ruling, focusing on the difference between an oath (sh'vuah) and a vow (neder). When taking an oath, a person causes his own person to be prohibited against performing a particular action. To use yeshivah terminology, it is an issur gavra; the prohibition is on the person. When taking a vow, by contrast, he places the prohibition on the object. It is an issur cheftzah.
Now when a person takes a vow against a colleague benefiting from his property, there is no difficulty, because he is placing the prohibition on the property. How can he, however, place a prohibition on a colleague's person? How can his oath take effect?
The Rambam's ruling is quoted by the Shulchan Aruch (Yoreh De'ah 236:3; albeit using slightly different wording). The Turei Zahav 236:7 explains that the Rambam follows the principal stated by the Ramban that an oath expressed using the wording of a vow and a vow expressed using the wording of an oath is binding. The Radbaz, puzzled by the same difficulty, states that this refers to an instance where the colleague answered Amen to the oath.
Similar laws apply in all analogous situations.

הלכה ה
שְׁבוּעָה שֶׁלֹּא אֹכַל וְאָכַל אֳכָלִין שֶׁאֵינָן רְאוּיִין לַאֲכִילָה וְשָׁתָה מַשְׁקִים שֶׁאֵינָן רְאוּיִין לִשְׁתִיָּה פָּטוּר. אָכַל דְּבָרִים הָאֲסוּרִין מִן הַתּוֹרָה בַּאֲכִילָה כְּגוֹן שֶׁאָכַל כְּזַיִת נְבֵלוֹת וּטְרֵפוֹת שְׁקָצִים וּרְמָשִׂים פָּטוּר מִשּׁוּם שְׁבוּעַת בִּטּוּי. שְׁבוּעָה שֶׁאֹכַל וְאָכַל אֳכָלִין שֶׁאֵינָן רְאוּיִים לַאֲכִילָה וְשָׁתָה מַשְׁקִין שֶׁאֵינָן רְאוּיִין לִשְׁתִיָּה אוֹ שֶׁאָכַל נְבֵלוֹת וּטְרֵפוֹת וְכַיּוֹצֵא בָּהֶן פָּטוּר מִשְּׁבוּעַת בִּטּוּי שֶׁהֲרֵי יָצָא יְדֵי אֲכִילָה מֵאַחַר שֶׁהֵן חֲשׁוּבִין אֶצְלוֹ אֲכִילָתָן שְׁמָהּ אֲכִילָה:
כסף משנה
5.
[When a person says: "I am taking] an oath that I will not eat," and he ate, but he ate articles that were not fit to be eaten10E.g., earth or spoiled foods. or drank beverages that were not fit to be drunk, he is exempt.11The Radbaz explains that he is eating them, not because he considers them as food, but in order to quench his pangs of hunger. If he partook of foods that are forbidden to be eaten by the Torah, for example, he ate an olive-sized portion of a nevelah,12An animal that died without ritual slaughter. a trefe,13An animal with a blemish that would cause it to die within twelve months. teeming animals, or creeping animals, he is not liable for a [false] sh'vuat bitui.14Since he is already forbidden to partake of these entities by the oath taken by the Jewish people as a whole at Sinai, the oath he takes is of no significance (Sh'vuot 22b). See Halachah 11.
The Radbaz emphasizes that this exclusion applies only with regard to entities forbidden by Scriptural Law, but not those forbidden by Rabbinic Law. For in such an instance, he is not bound by the oath taken by our people at Sinai.

[When a person says: "I am taking] an oath that I will eat," and he ate articles that were not fit to be eaten or drank beverages that were not fit to be drunk, or he partook of a nevelah, a trefe, or the like, he is not liable for a false sh'vuat bitui. He is considered to have fulfilled [his commitment to] eat. Since they are important in his eyes, eating them is considered as eating.15Rabbenu Nissim explains the difference between this and the first clause as follows: In the first clause, we assume that the not eating, he referred to in his oath was not eating foods that people usually eat. These articles were not included in his oath, for there is no reason to forbid them. In the second instance, he included everything that he considers as food in his oath.

הלכה ו
שְׁבוּעָה שֶׁלֹּא אָכַלְתִּי וְהוּא אָכַל דְּבָרִים שֶׁאֵינָן רְאוּיִין לַאֲכִילָה אוֹ נְבֵלוֹת וּטְרֵפוֹת חַיָּב שֶׁאֲכִילָתָן שְׁמָהּ אֲכִילָה שֶׁכְּבָר הֵן חֲשׁוּבִין אֶצְלוֹ שֶׁהֲרֵי אֲכָלָן. אֲבָל לְהַבָּא שֶׁנִּשְׁבַּע שֶׁלֹּא יֹאכַל וְנִקְרָה מִקְרֶה וַאֲכָלָן אֵינָהּ אֲכִילָה כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
6.
[When a person said: "I am taking] an oath that I did not eat," and he ate articles that were not fit to be eaten or he partook of a nevelah or a trefe, he is liable. Eating them is considered eating, because they are important to him, as evidenced by his having eaten them.16Even before he took his oath. With regard to the future, by contrast, i.e., he took an oath that he would not eat and then in an extraordinary instance, he ate them, this is not considered eating, as we explained [above].

הלכה ז
שְׁבוּעָה שֶׁלֹּא אֹכַל כָּל שֶׁהוּא מִנְּבֵלוֹת וּטְרֵפוֹת וְאָכַל פָּחוֹת מִכְּזַיִת חַיָּב בִּשְׁבוּעָה שֶׁהֲרֵי אֵינוֹ מֻשְׁבָּע עַל חֲצִי שִׁעוּר מֵהַר סִינַי:
כסף משנה
7.
[When a person says: "I am taking] an oath that I will not eat even the slightest amount of a nevelah or a trefe," and he ate less than an olive-sized portion, he is liable for taking a [false] oath, for he is not bound by an oath from Mount Sinai17As the Torah states: "Cursed is the man who will not observe the words of this Torah" (Deuteronomy 27:26 . for half the measure [which makes him liable].18The Radbaz explains that although the Rambam maintains that there is a Scriptural prohibition against eating even less than the measure for which one is liable (Hilchot Ma'achalot Asurot 14:2), this is not considered a matter for which one is bound by an oath from Sinai. For that oath includes only those matters which are explicitly mentioned by the Torah and this prohibition is not. There are, however, other Rishonim who do not makes such a distinction. See Siftei Cohen 238:6.

הלכה ח
שְׁבוּעָה שֶׁאֹכַל פָּחוֹת מִכְּזַיִת נְבֵלָה וּטְרֵפָה חַיָּב בִּשְׁבוּעַת בִּטּוּי. שְׁבוּעָה שֶׁלֹּא אֹכַל עָפָר וְכַיּוֹצֵא בּוֹ מִדְּבָרִים שֶׁאֵינָן רְאוּיִין לַאֲכִילָה אִם אָכַל כְּזַיִת חַיָּב. אָכַל פָּחוֹת מִכְּזַיִת הֲרֵי זֶה סָפֵק שֶׁמָּא יִתְחַיֵּב בְּכָל שֶׁהוּא מִפְּנֵי שֶׁאֵין זֶה דַּרְכּוֹ לַאֲכִילָה כְּדֵי לְהַצְרִיכוֹ שִׁעוּר:
כסף משנה
8.
[When a person says: "I am taking] an oath that I will eat even less than an olive-sized portion of a nevelah or a trefe," he may be liable for taking a false sh'vuat bitui.19The oath takes effect, because, as stated in the previous halachah, for this quantity, he is not bound by an oath from Sinai. The Radbaz states that preferably, he should have this oath nullified. Nevertheless, if that is not possible, it is preferable for him to keep the oath and violate the Scriptural commandment.
[When a person says: "I am taking] an oath that I will not eat earth and the like from substances that are not fit to be eaten," if he eats an olive-sized portion, he is liable. If he ate less than an olive-sized portion, there is a doubt [concerning the ruling]. Perhaps he is liable even for [eating] the smallest amount. Since these substances are not usually eaten so that a full measure must be eaten [for him to be held liable].20On the other hand, perhaps, he is not liable, for since he mentioned "eating" in his oath, we assumed that he meant an olive-sized portion.

הלכה ט
וְכֵן הַנִּשְׁבָּע שֶׁלֹּא לֶאֱכל חַרְצָן וְאָכַל מִמֶּנּוּ פָּחוֹת מִכְּזַיִת הֲרֵי זֶה סָפֵק. הָיָה הַנִּשְׁבָּע נָזִיר שֶׁהוּא אָסוּר בְּחַרְצָן בִּכְזַיִת וְאָכַל מִמֶּנּוּ פָּחוֹת מִכְּזַיִת אֵינוֹ חַיָּב בִּשְׁבוּעַת בִּטּוּי. שֶׁהֲרֵי אֵין דַּעְתּוֹ בִּשְׁבוּעָתוֹ אֶלָּא עַל (כָּל) כְּזַיִת שֶׁהוּא מֻשְׁבָּע עָלָיו וְאֵין שְׁבוּעָה חָלָה עָלָיו. לְפִיכָךְ אִם פֵּרֵשׁ וְאָמַר שְׁבוּעָה שֶׁלֹּא אֹכַל חַרְצָן אֶחָד וַאֲכָלוֹ חַיָּב:
כסף משנה
9.
Similarly, when one takes an oath that he would not eat grape seeds and he eats less than an olive-sized portion, there is a doubt [concerning his liability].21Perhaps he is liable for, as mentioned above, since such articles are not usually eaten, he may be held liable even for eating less than the usual amount or perhaps we require an olive-sized portion. If the one taking the oath was a nazirite who is forbidden to eat an olive-sized portion of grape seeds,22As stated in Numbers 6:4. he is not liable for a [false] sh'vuat bitui if he ate less than an olive-sized portion. [The rationale is that] his intent in taking the oath is only concerning the olive-sized portion for which he is already liable and [hence] the oath does not take effect.23As stated in Halachah 11. Therefore if one said: "[I am taking] an oath that I will not eat even one grape seed," and ate it, he is liable.24See Chapter 4, Halachah 1.

הלכה י
שְׁבוּעָה שֶׁלֹּא אֹכַל תְּמָרִים וּנְבֵלוֹת וּטְרֵפוֹת וְאָכַל כְּזַיִת נְבֵלָה אוֹ טְרֵפָה חַיָּב אַף מִשּׁוּם שְׁבוּעַת בִּטּוּי שֶׁהֲרֵי כָּלַל דְּבָרִים הָאֲסוּרִין עִם דְּבָרִים הַמֻּתָּרִין. וּמִתּוֹךְ שֶׁחָלָה שְׁבוּעָה עַל הַתְּמָרִים חָלָה עַל דְּבָרִים הָאֲסוּרִים כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
10.
[When a person says: "I am taking] an oath that I will not eat dates, a nevelah or a trefe," and he ate an olive-sized portion of a nevelah or a trefe, he is liable also25I.e., in addition to violating the prohibition against forbidden foods. for [taking] a [false] sh'vuat bitui.26We do not say he is required to eat the two together. For he included forbidden entities together with permitted entities. Since the oath took effect with regard to the dates, it also takes effect with regard to the forbidden entities, as we explained.27Chapter 4, Halachah 11.

הלכה יא
אֲבָל אִם נִשְׁבַּע שֶׁלֹּא יֹאכַל נְבֵלָה וּטְרֵפָה וְכַיּוֹצֵא בָּהֶן בֵּין אָכַל בֵּין לֹא אָכַל אֵין כָּאן חִיּוּב שְׁבוּעָה כְּלָל לֹא שְׁבוּעַת בִּטּוּי וְלֹא שְׁבוּעַת שָׁוְא:
כסף משנה
11.
If, however, a person took an oath that he would not eat a nevelah, a trefe, or the like,28Substances explicitly forbidden by the Torah. regardless of whether he partook of [the forbidden substance] or not, there is no obligation for an oath at all, neither a sh'vuat bitui,29This oath does not take effect, because an oath cannot take effect with regard to an object bound by another oath. Since the entire Jewish people are bound by the oath taken at Sinai not to partake of these substances, no other oath involving these entities can take effect (Kessef Mishneh). nor an oath taken in vain.30Were the person to have taken an oath to eat the forbidden substance, he would be taking an oath in vain, for his oath would be to nullify one of the Torah's mitzvot. In this instance, however, he is taking an oath to fulfill the mitzvah. This is permitted. See Nedarim 8b; Chapter 11, Halachah 3.

הלכה יב
נִשְׁבַּע שֶׁיֹּאכַל נְבֵלָה וּטְרֵפָה וְכַיּוֹצֵא בָּהֶן מֵאִסּוּרֵי תּוֹרָה הֲרֵי זֶה לוֹקֶה מִשּׁוּם שְׁבוּעַת שָׁוְא בֵּין אָכַל בֵּין לֹא אָכַל:
כסף משנה
12.
When a person takes an oath that he will partake of a nevelah, a trefe, or another similar substance forbidden by the Torah, he is liable for lashes for taking an oath in vain31See Chapter 1, Halachah 6. whether he partook of the substance or not.32For the oath is considered as having been taken in vain at the moment it was uttered (see Rashi, Sh'vuot 29b).

הלכה יג
שְׁבוּעָה שֶׁאֹכַל כִּכָּר זוֹ שְׁבוּעָה שֶׁלֹּא אֹכָלֶנָּה הַשְּׁנִיָּה שְׁבוּעַת שָׁוְא שֶׁהֲרֵי מִצְוָה עָלָיו לְאָכְלָהּ וְלוֹקֶה עַל שְׁבוּעָה שְׁנִיָּה בֵּין אָכַל בֵּין לֹא אָכַל. וְאִם לֹא אֲכָלָהּ חַיָּב אַף בִּשְׁבוּעַת בִּטּוּי:
כסף משנה
13.
[When a person says: "I am taking] an oath that I will eat this loaf. [I am taking] an oath that I will not eat it," the second oath is an oath taken in vain, for he is commanded to eat it.33Due to his first oath. He is liable for lashes for the second oath whether he partakes of [the loaf] or not.34As stated in the previous halachah. If he does not eat it,35I.e., within the time period he specified in the oath; alternatively, after the loaf was destroyed or eaten by others. As long as the loaf continues to exist, however, he may fulfill his oath. he is liable also for [not fulfilling] a sh'vuat bitui.36For his first oath is binding.

הלכה יד
שְׁבוּעָה שֶׁלֹּא אֹכַל כִּכָּר זוֹ שְׁבוּעָה שֶׁאֹכָלֶנָּה. הַשְּׁנִיָּה שְׁבוּעַת שָׁוְא שֶׁהֲרֵי זוֹ אֲסוּרָה עָלָיו לְאָכְלָהּ וְלוֹקֶה עַל שְׁבוּעָה שְׁנִיָּה זוֹ בֵּין אֲכָלָהּ בֵּין לֹא אֲכָלָהּ. וְאִם אֲכָלָהּ לוֹקֶה אַף מִשּׁוּם שְׁבוּעַת בִּטּוּי. וְכֵן כָּל הַנִּשְׁבָּע לְבַטֵּל אֶת הַמִּצְוָה וְלֹא בִּטֵּל פָּטוּר מִשְּׁבוּעַת בִּטּוּי וְלוֹקֶה מִשּׁוּם שְׁבוּעַת שָׁוְא וְעוֹשֶׂה הַמִּצְוָה שֶׁנִּשְׁבַּע לְבַטְּלָהּ:
כסף משנה
14.
[When a person says: "I am taking] an oath that I will not eat this loaf. [I am taking] an oath that I will eat it," the second oath is an oath taken in vain, for he is forbidden to eat it.37Due to his first oath, as above. He is liable for lashes for the second oath whether he partakes of [the loaf] or not. If he eats it, he is liable also for [not fulfilling] a sh'vuat bitui.
Similarly, whenever one takes an oath to neglect a mitzvah and does not neglect it, he is exempt for [violating] a sh'vuat bitui.38For the reasons stated in Halachah 11. He is, however, liable for lashes for taking an oath in vain.39As stated in Chapter 1, Halachah 6. He should perform the mitzvah that he took an oath to neglect.

הלכה טו
כֵּיצַד. כְּגוֹן שֶׁנִּשְׁבַּע שֶׁלֹּא יַעֲשֶׂה סֻכָּה וְשֶׁלֹּא יִלְבַּשׁ תְּפִלִּין וְשֶׁלֹּא יִתֵּן צְדָקָה. וְכֵן הַנִּשְׁבָּע לַחֲבֵרוֹ שֶׁלֹּא אָעִיד לְךָ עֵדוּת זוֹ שֶׁאֲנִי יוֹדְעָהּ אוֹ שֶׁלֹּא אָעִיד לְךָ אִם אֵדַע לְךָ עֵדוּת הֲרֵי זֶה לוֹקֶה מִשּׁוּם שְׁבוּעַת שָׁוְא מִפְּנֵי שֶׁהוּא מְצֻוֶּה לְהָעִיד. וְכֵן הָאוֹמֵר לַחֲבֵרוֹ שְׁבוּעָה שֶׁלֹּא אֵדַע לְךָ עֵדוּת הֲרֵי זוֹ שְׁבוּעַת שָׁוְא שֶׁאֵין בְּיָדוֹ שֶׁלֹּא יֵדַע לוֹ עֵדוּת. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה:
כסף משנה
15.
What is implied? For example, a person took an oath that he would not make a sukkah, he would not put on tefillin, he would not give charity, he is liable for lashes for taking an oath in vain.40For these are all mitzvot that he is required to fulfill. Similarly, [one is liable] if he takes an oath for a colleague that he will not give testimony that he knows or that he will not testify if he will know testimony, for he is commanded to testify.41Sefer HaMitzvot (positive commandment 178) and Sefer HaChinuch (mitzvah 122) count this as one of the 613 mitzvot of the Torah. See Hilchot Edut 1:1. Similarly, if he tells a colleague: "[I am taking] an oath that I will never know testimony concerning you," it is an oath taken in vain, for it is not within his capacity [to be certain] that he will never know of testimony concerning him. Similar laws apply in all analogous situations.

הלכה טז
נִשְׁבַּע לְקַיֵּם אֶת הַמִּצְוָה וְלֹא קִיֵּם פָּטוּר מִשּׁוּם שְׁבוּעַת בִּטּוּי. כֵּיצַד. כְּגוֹן שֶׁנִּשְׁבַּע שֶׁיַּעֲשֶׂה לוּלָב אוֹ סֻכָּה אוֹ שֶׁיִּתֵּן צְדָקָה לְעָנִי אוֹ שֶׁיָּעִיד לוֹ אִם יֵדַע לוֹ עֵדוּת וְלֹא עָשָׂה וְלֹא נָתַן וְלֹא הֵעִיד הֲרֵי זֶה פָּטוּר מִשּׁוּם שְׁבוּעַת בִּטּוּי. שֶׁאֵין שְׁבוּעַת בִּטּוּי חָלָה אֶלָּא עַל דִּבְרֵי הָרְשׁוּת שֶׁאִם רָצָה עוֹשֶׂה וְאִם לֹא רָצָה אֵינוֹ עוֹשֶׂה שֶׁנֶּאֱמַר (ויקרא ה ד) "לְהָרַע אוֹ לְהֵיטִיב". לְפִיכָךְ כָּל הַנִּשְׁבָּע לְהָרַע לַאֲחֵרִים פָּטוּר מִשְּׁבוּעַת בִּטּוּי כְּגוֹן שֶׁנִּשְׁבַּע שֶׁיַּכֶּה אֶת פְּלוֹנִי אוֹ יְקַלְּלֶנּוּ אוֹ יִגְזל מָמוֹנוֹ אוֹ יִמְסְרֶנּוּ בְּיַד אַנָּס מִפְּנֵי שֶׁהוּא מְצֻוֶּה שֶׁלֹּא לַעֲשׂוֹת. וְיֵרָאֶה לִי שֶׁהוּא לוֹקֶה מִשּׁוּם שְׁבוּעַת שָׁוְא:
כסף משנה
16.
When a person takes an oath to fulfill a mitzvah and fails to fulfill it, he is not liable for not fulfilling a sh'vuat bitui.42Nor is he liable for taking an oath in vain, for at the time he took the oath, it was not in vain. And one may take an oath to observe the mitzvot, so his intent was desirable (Radbaz).
What is implied? A person took an oath to make a lulav or a sukkah, to give charity, or to testify on behalf [of a colleague] if he knew testimony [that could affect him]. If he did not make [these articles], give [the charity], or testify, he is exempt for [not fulfilling his] sh'vuat bitui. For a sh'vuat bitui takes effect only with regard to matters left to one's choice - [i.e., matters that] if he wants to, he may perform and if he does not want to, he need not perform, as implied by [Leviticus 5:4]: "whether he will do harm or do good."
Therefore whenever anyone takes an oath to harm another person, he is exempt from a sh'vuat bitui, e.g., he takes an oath to strike so-and-so, to curse him, steal his money, or deliver him to the control of a man of force. [The rationale is that] he is commanded not to do [these things]. It appears to me that he is liable for lashes for taking an oath in vain.43I.e., since performing any one of these acts violates one of the Torah's prohibitions, taking an oath to perform such an act is equivalent to taking an oath to nullify a mitzvah. Nevertheless, the Rambam prefaces his ruling with the words "It appears to me" - which indicate a ruling based on his own deductive processes - for, in prior Rabbinic sources, the statement that taking an oath to nullify a mitzvah is considered taking an oath in vain were made with regard to prohibitions between man and God and these are prohibitions between man and man.

הלכה יז
נִשְׁבַּע לְהָרַע לְעַצְמוֹ כְּגוֹן שֶׁנִּשְׁבַּע שֶׁיַּחֲבל בְּעַצְמוֹ אַף עַל פִּי שֶׁאֵינוֹ רַשַּׁאי שְׁבוּעָה חָלָה עָלָיו. וְאִם לֹא הֵרַע לְעַצְמוֹ חַיָּב מִשּׁוּם שְׁבוּעַת בִּטּוּי. נִשְׁבַּע לְהֵיטִיב לַאֲחֵרִים הֲטָבָה שֶׁבְּיָדוֹ לַעֲשׂוֹתָהּ כְּגוֹן שֶׁיְּדַבֵּר עָלָיו לַשִּׁלְטוֹן אוֹ יְכַבְּדֶנּוּ שְׁבוּעָה חָלָה עָלָיו. וְאִם עָבַר וְלֹא עָשָׂה חַיָּב מִשּׁוּם שְׁבוּעַת בִּטּוּי:
כסף משנה
17.
If a person took an oath to harm himself, e.g., he took an oath to inflict injury upon himself, the oath takes effect even though he is not allowed to do so.44Hilchot Chovel UMazik 5:1 states that a person may not injure himself. Nevertheless, since this prohibition is not explicitly stated in the Torah, it is not considered as one is taking an oath to nullify a mitzvah (see Halachah 7) and the oath takes effect (Radbaz). If he does not harm himself, he is liable for [not fulfilling] a sh'vuat bitui.
If he took an oath to help others with regard to a matter with which he could help them,45If, however, it is not in his capacity to perform this favor, he is liable for taking an oath in vain, but not for failing to fulfill a sh'vuat bitui (Radbaz).
Performing deeds of kindness fulfills a mitzvah. Nevertheless, since the specific deeds are not explicitly mentioned in the Torah as mitzvot, the violation of an concerning them is considered as a false sh'vuat bitui.
e.g., to speak to the ruling authorities or to show him honor, the oath takes effect. If he transgresses and does not carry out [his promise], he is liable for [not fulfilling] a sh'vuat bitui.

הלכה יח
נִשְׁבַּע שֶׁלֹּא יֹאכַל מַצָּה שָׁנָה אוֹ שְׁנָתַיִם. הֲרֵי זֶה אָסוּר לֶאֱכל מַצָּה בְּלֵילֵי הַפֶּסַח וְאִם אָכַל חַיָּב מִשּׁוּם שְׁבוּעַת בִּטּוּי. וְאֵין זֶה שְׁבוּעַת שָׁוְא שֶׁהֲרֵי לֹא נִשְׁבַּע שֶׁלֹּא יֹאכַל מַצָּה בְּלֵילֵי הַפֶּסַח אֶלָּא כָּלַל עִתִּים שֶׁאֲכִילַת מַצָּה בָּהֶם רְשׁוּת עִם עֵת שֶׁאֲכִילָתָהּ בּוֹ מִצְוָה וּמִתּוֹךְ שֶׁחָלָה שְׁבוּעָה עַל שְׁאָר הַיָּמִים חָלָה עַל לֵיל הַפֶּסַח. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה. כְּגוֹן שֶׁנִּשְׁבַּע שֶׁלֹּא יֵשֵׁב בְּצֵל סֻכָּה לְעוֹלָם אוֹ שֶׁלֹּא יַעֲלֶה עָלָיו בֶּגֶד שָׁנָה אוֹ שְׁנָתַיִם:
כסף משנה
18.
One who takes an oath not to eat matzah for a year or two is forbidden to eat matzah on the nights of Pesach.46When we are commanded to eat matzah. The mitzvah applies only on the night of the fifteenth of Nisan and not throughout the holiday. If he eats it, he is liable, for violating a sh'vuat bitui. This is not considered as an oath taken in vain, since he did not take an oath [specifically] not to eat matzah on the nights of Pesach. Instead, he included the times when eating matzah is a matter of choice together with those when it is a mitzvah. Since the oath takes effect with regard to the other days, it also takes effect with regard to Pesach. Similar laws apply in all analogous situations,47The Rama (Yoreh De'ah 236:5, quoting the Maharam of Padua, Responsa 74) emphasizes that this ruling only applies with regard to positive commandments, but not with regard to the Torah's prohibitions. Thus if a person took an oath that he would eat all types of meat, we do not say that since the oath takes effect with regard to the kosher meat, it also takes effect with regard to the non-kosher meat. e.g., one took an oath not to sit in the shade of a sukkah forever,48And thus the oath also prevents one from fulfilling the mitzvah of dwelling in a sukkah on Sukkot. or not to wear a garment for a year or two.49The Radbaz interprets the oath as preventing the person from fulfilling the mitzvah of tzitzit. Nevertheless, as the Radbaz himself notes, this interpretation is somewhat problematic, because there is no Scriptural mitzvah to wear tzitzit each day. Instead, the mitzvah is that if one is wearing a four-cornered garment, one must attach tzitzit to it. See Hilchot Tzitzi 3:11. Others interpret this as referring to priests who take such an oath and thus are prevented from wearing the priestly garments while serving in the Temple. As stated in Hilchot Klei HaMikdash 10:4, wearing such garments is a mitzvah.

הלכה יט
נִשְׁבַּע שֶׁהֵנִיחַ תְּפִלִּין הַיּוֹם אוֹ לֹא הֵנִיחַ. שֶׁנִּתְעַטֵּף בְּצִיצִית אוֹ לֹא נִתְעַטֵּף. הֲרֵי זֶה שְׁבוּעַת בִּטּוּי לְשֶׁעָבַר. שֶׁזֶּה מַגִּיד דָּבָר שֶׁנַּעֲשָׂה וְלֹא נִשְׁבַּע עַל הַמִּצְוָה לֹא לְקַיְּמָהּ וְלֹא לְבַטְּלָהּ:
כסף משנה
19.
If one took an oath that he put on tefillin that day or did not put them on, or wrapped himself in tzitzit or did not wrap himself in them, he is taking a sh'vuat bitui with regard to the past.50And he is liable if the oath is false. For he is describing something which happened. He is not taking an oath whether to fulfill or not to fulfill a mitzvah.

הלכה כ
נִשְׁבַּע שֶׁלֹּא יִישַׁן שְׁלֹשֶׁת יָמִים. אוֹ שֶׁלֹּא יֹאכַל כְּלוּם שִׁבְעַת יָמִים וְכַיּוֹצֵא בָּזֶה שֶׁהִיא שְׁבוּעַת שָׁוְא. אֵין אוֹמְרִין יֵעוֹר זֶה עַד שֶׁיִּצְטַעֵר וְיָצוּם עַד שֶׁיִּצְטַעֵר וְלֹא יִהְיֶה בּוֹ כֹּחַ לִסְבּל וְאַחַר כָּךְ יֹאכַל אוֹ יִישַׁן. אֶלָּא מַלְקִין אוֹתוֹ מִיָּד מִשּׁוּם שְׁבוּעַת שָׁוְא וְיִישַׁן וְיֹאכַל בְּכָל עֵת שֶׁיִּרְצֶה:
כסף משנה
20.
If a person took an oath that he will not sleep for a three-day period, he will not eat for seven days, or the like, it is an oath taken in vain.51For there is no way that he can keep his word. Thus from the moment he uttered the oath, it was uttered in vain (Radbaz). See Chapter 1, Halachah 7. We do not say that the person should remain awake until he is overcome by pain or fast until he is overcome by pain and [only] when he no longer has the strength to bear [the suffering], eat or sleep.52The Kessef Mishneh quotes Rabbenu Nissim who questions the similarity between the two instances. It is impossible that a person will not sleep for seven days. He will fall asleep whether he desires to or not. Hence, he should not even try to remain awake. With regard to eating, by contrast, seemingly, the person should wait until he reaches a dangerous state and then he should be allowed to eat.
Based on the commentary of the Tzaphnat Paneach, it is possible to explain the differences in approach as follows: According to Rabbenu Nissim, the prohibition is lifted because of the danger, but it is not nullified entirely. Hence, when a person takes an oath on a matter that involves danger, we lift the prohibition, but only after we have waited until the danger is acutely felt. Hence, the oath not to eat is not necessarily a false oath. The oath not to sleep, by contrast, is definitely false, because it is impossible that he will not sleep.
According to the Rambam, by contrast, since there is danger to life involved, the prohibition is nullified entirely. Hence, even the oath not to eat is considered to have been taken in vain.
Instead, he is liable for lashes53Our translation is based on the commentary of the Radbaz. Even if there is no court to administer this punishment to him, he may eat and sleep whenever he desires. When he is brought before the court, they will subject him to punishment. immediately for taking an oath in vain. He may eat and sleep whenever he desires.54For the oath is not considered to have taken effect at all.

הלכה כא
נִשְׁבַּע שֶׁרָאָה גָּמָל פּוֹרֵחַ בָּאֲוִיר. אָמְרוּ לוֹ הֵיאַךְ נִשְׁבַּעְתָּ לַשָּׁוְא וְאָמַר עוֹף גָּדוֹל רָאִיתִי וּמִגָּדְלוֹ קְרָאתִיו גָּמָל וְכָךְ הָיָה בְּדַעְתִּי אֵין זֶה כְּלוּם. שֶׁאֵין כָּל אָדָם קוֹרְאִין גָּמָל אֶלָּא לְגָמָל וּבָטְלָה דַּעְתּוֹ אֵצֶל כָּל אָדָם וְלוֹקֶה. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה:
כסף משנה
21.
When a person takes an oath that he saw a camel flying in the sky and when questioned: "How could you have taken an oath in vain?", he responded: "I saw a huge bird and because of its size, I called it a camel. This was my intent," [his words] are of no consequence. For when all people mention a camel that is their intent. His intention is nullified because of that of people at large55Because the meaning of phrases used by people at large determines the ruling with regard to oaths and vows (Radbaz). and he is liable for lashes.56For taking an oath in vain. Similar laws apply in all analogous situations.

הלכה כב
דָּבָר יָדוּעַ אֵצֶל הַחֲכָמִים בַּעֲלֵי שֵׂכֶל וּמַדָּע שֶׁהַשֶּׁמֶשׁ גְּדוֹלָה מִן הָאָרֶץ מֵאָה וְשִׁבְעִים פְּעָמִים. נִשְׁבַּע אֶחָד מִן הָעָם שֶׁהַשֶּׁמֶשׁ גְּדוֹלָה מִן הָאָרֶץ אֵינוֹ לוֹקֶה מִשּׁוּם שְׁבוּעַת שָׁוְא שֶׁאַף עַל פִּי שֶׁהַדָּבָר כֵּן הוּא אֵין דָּבָר זֶה גָּלוּי וְיָדוּעַ לְכָל הָעָם אֶלָּא לִגְדוֹלֵי הַחֲכָמִים בִּלְבַד. וְאֵינוֹ חַיָּב אֶלָּא אִם כֵּן נִשְׁבַּע עַל דָּבָר שֶׁגָּלוּי וְיָדוּעַ לִשְׁלֹשָׁה בְּנֵי אָדָם מִשְּׁאָר הָעָם כְּגוֹן אִישׁ שֶׁהוּא אִישׁ וְאֶבֶן שֶׁהוּא אֶבֶן. וְכֵן אִם נִשְׁבַּע שֶׁהַשֶּׁמֶשׁ קְטַנָּה מִן הָאָרֶץ אֵינוֹ לוֹקֶה. וְאַף עַל פִּי שֶׁאֵין הַדָּבָר כֵּן מִפְּנֵי שֶׁאֵין זֶה יָדוּעַ לְכָל אָדָם וְאֵינוֹ כְּמִי שֶׁנִּשְׁבַּע עַל הָאִישׁ שֶׁהוּא אִשָּׁה שֶׁהֲרֵי לֹא נִשְׁבַּע אֶלָּא עַל רְאִיַּת עֵינָיו שֶׁהֲרֵי הוּא רוֹאֶה אוֹתָהּ קְטַנָּה. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה מִדִּבְרֵי חֶשְׁבּוֹן תְּקוּפוֹת מַזָּלוֹת וְגִימַטְרִיָּאוֹת וְכַיּוֹצֵא בָּהֶן מִדִּבְרֵי חָכְמָה שֶׁאֵין נִכָּרִין אֶלָּא לַאֲנָשִׁים אֲחֵרִים:
כסף משנה
22.
It is a known matter to the sages who are masters of wisdom and knowledge that the sun is 170 times greater than the earth.57Actually, according to the scientific data available at present, the sun is far larger than this. Some have tried to reconcile the Rambam's statements with this data by explaining that the Rambam is speaking about the actual mass of the sun and not the burning energy on its surface. See Likkutei Sichot, Vol. 10, p. 180. [Nevertheless,] if one of the common people takes an oath that the sun is greater than the earth, he is not liable for taking an oath in vain.58I.e., one might think that since this is the reality, taking such an oath is considered an oath in vain. The Rambam is clarifying that since people at large may not be aware of this fact, it is not placed in that category. For even though this is the fact, this concept is not known to people at large, only to great sages. One is liable [for an oath taken in vain] only when he takes an oath concerning a matter that is known and obvious to three ordinary people, e.g., [an oath that] a man is a man or a stone is a stone.
Similarly, when he takes an oath that the sun is smaller than the earth, he is not liable for lashes [for an oath taken in vain] although this is not the reality. For this matter is not known to all people.59The Radbaz states that even if the person taking the oath knows that the sun is larger than the earth, he is not liable for taking an oath that is smaller, for people at large do not know this fact. Such a person is not comparable to one who takes an oath that a man is a woman. For he took the oath according to his perception, for the sun looks small. Similar laws apply to other comparable concepts from the reckoning of the factors determining the calendar, astronomy, geometry, and other abstract concepts of the like that can be perceived only by other people.

הפלאה הלכות שבועות פרק ה
Haflaah Shevuos Chapter 5