Public Vows and Defining the Firstborn Son

How the Talmud defines irreversible public oaths and distinguishes between primogeniture for inheritance and the redemption of the firstborn.

Vows on Public Consent

A kohen who contracted a prohibited marriage is disqualified from performing the Temple service until he severs the union. If his transgression is discovered mid-service, the Gemara requires him to vow against deriving benefit from his wife before proceeding. To prevent him from later seeking halakhic annulment, the court administers the vow על דעת רבים (upon public consent). Ameimar establishes the decisive halacha: a vow taken on public consent cannot be dissolved except when annulment is required for a דבר מצוה (a mitzvah purpose). Unlike a priest exposed to corpse impurity, who requires only a verbal commitment, a forbidden marriage demands an unbreakable vow because his illicit desire overpowers him.

The Rambam codifies that bet din compels the priest to vow על דעת רבים so that it has no annulment, after which he finishes serving, descends, and divorces her. In the laws of oaths, the Rambam confirms that any vow taken on public consent cannot be released except for a mitzvah. Maran rules in the Shulchan Arukh that vows on public consent cannot be dissolved without the public's consent unless the annulment serves a mitzvah. The Rema adds that some authorities prohibit dissolution even with public consent for discretionary matters, which one must follow strictly lechatchilah.

הִלְכְתָא, אֲפִילּוּ לְמַאן דְּאָמַר נֶדֶר שֶׁהוּדַּר בָּרַבִּים יֵשׁ לוֹ הֲפָרָה, עַל דַּעַת הָרַבִּים אֵין לוֹ הֲפָרָה. וְהָנֵי מִילֵּי לִדְבַר הָרְשׁוּת, אֲבָל לִדְבַר מִצְוָה יֵשׁ לוֹ הֲפָרָה.

The halakha is: Even according to the one who says a vow taken in public has nullification, on public consent it has no nullification. And this applies only to an optional matter, but for a mitzvah purpose it has nullification. (Bekhorot 46a:2)

Inheritance Versus Priestly Redemption

The Mishnah establishes four categories distinguishing a בכור לנחלה (firstborn for inheritance) from a בכור לכהן (firstborn for redemption). Priestly redemption depends exclusively on maternal status as a פטר רחם (the opening of the womb). By contrast, the inheritance right to a double portion belongs solely to the father's first viable issue, described by the Torah as ראשית אונו (the beginning of his strength). Consequently, a son born after non-viable miscarriages or after a stillborn whose head emerged dead remains a firstborn for inheritance while being entirely exempt from priestly redemption.

The Tur and Shulchan Arukh codify that a son born after miscarriages—even if the head of the non-viable fetus emerged alive—or after a nine-month fetus whose head emerged dead, inherits a double portion. Because a father does not grieve over a non-viable offspring, an unviable fetus does not deprive the subsequent brother of his status as the father's firstborn. Only when a viable nine-month infant emerges with most of its head alive does it disqualify a subsequent brother from claiming the double portion.

הבא אחר נפלים אע"פ שיצא ראש הנפל כשהוא חי הבא אחריו בכור לנחלה וכן בן ט' שיצא ראשו מת הבא אחריו בכור לנחלה

One who follows miscarriages, even if the head of the non-viable fetus emerged alive, the one born after it is a firstborn for inheritance; and likewise a nine-month fetus whose head emerged dead, the one born after it is a firstborn for inheritance. (Shulchan Arukh, Choshen Mishpat 277:6)

Defining Birth by the Forehead

The Gemara examines the precise anatomical threshold that constitutes legal birth when an infant emerges headfirst. Rabbi Shimon ben Lakish maintains that the emergence of the פדחת (forehead) defines birth in all legal contexts except inheritance, which requires full facial recognition to satisfy the biblical term יכיר (he shall recognize). Rabbi Yoḥanan disagrees, ruling that the emergence of the forehead establishes birth even for inheritance.

The halakha follows Rabbi Yoḥanan over Rabbi Shimon ben Lakish. The Tur rules accordingly that if the majority of an infant's forehead emerged while the father was still alive, the child is considered fully born and secures his double portion, even if his entire head has not yet emerged from the birth canal.

Vows taken on public consent cannot be dissolved except for a mitzvah, while primogeniture for inheritance requires a viable birth defined from the moment the forehead emerges.

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