The First of His Vigor Across Conversion

How the Torah's biological definition of the firstborn prevents a convert from conferring a double inheritance upon his post-conversion son.

כִּי֩ אֶת־הַבְּכֹ֨ר בֶּן־הַשְּׂנוּאָ֜ה יַכִּ֗יר לָ֤תֶת לוֹ֙ פִּ֣י שְׁנַ֔יִם בְּכֹ֥ל אֲשֶׁר־יִמָּצֵ֖א ל֑וֹ כִּי־הוּא֙ רֵאשִׁ֣ית אֹנ֔וֹ ל֖וֹ מִשְׁפַּ֥ט הַבְּכֹרָֽה׃ {ס}

Instead, he must accept the first-born, the son of the unloved one, and allot to him a double portioncdouble portion Lit. two-thirds. of all he possesses; since he is the first fruit of his vigor, the birthright is his due. (Deuteronomy 21:17)

The Biological Claim to the Double Portion

In the Torah's laws of inheritance, a father must grant a double portion to his firstborn son, even if the child was born to an unloved wife. Rashi explains that the phrase "פי שנים" means an allotment equivalent to two brothers. Ibn Ezra notes that the verb "יכיר" requires the father to formally acknowledge and confirm the true firstborn before the community.

The Torah provides an explicit rationale for this mandate: "for he is the first of his vigor, the birthright is his due". Rashbam interprets this phrase to mean that this son is entitled to the primary inheritance because he represents the very first expenditure of his father's strength and wealth.

כִּֽי־הוּא֙ רֵאשִׁ֣ית אֹנ֔וֹ ל֖וֹ מִשְׁפַּ֥ט הַבְּכֹרָֽה

Since he is the first fruit of his vigor, the birthright is his due. (Deuteronomy 21:17)

The Convert and the Clean Slate

The Gemara raises a fundamental question concerning a gentile who fathered sons, subsequently converted to Judaism, and then fathered another son. Does this newly born son inherit a double portion as a בכור לנחלה (firstborn for inheritance)?

Rabbi Shimon ben Lakish rules that this son is indeed a firstborn for inheritance. He relies on the classic halachic principle that a convert is viewed as a newborn child ("גר שנתגייר כקטן שנולד דמי"). Because conversion completely severs past legal relationships, the sons born during his gentile life are disregarded, making the child born after conversion his legal firstborn.

וְרַבִּי שִׁמְעוֹן בֶּן לָקִישׁ אוֹמֵר: יֵשׁ לוֹ בְּכוֹר לַנַּחֲלָה — גֵּר שֶׁנִּתְגַּיֵּיר כְּקָטָן שֶׁנּוֹלַד דָּמֵי.

And Rabbi Shimon ben Lakish says: He does have a firstborn with regard to inheritance, as the halakhic status of a convert who just converted is like that of a child just born. (Bekhorot 47a:2)

Physical Vigor Versus Legal Status

Rabbi Yoḥanan disagrees with this reasoning and rules that the convert has no firstborn for inheritance. He grounds his ruling in the strict language of Deuteronomy 21:17, which defines the firstborn as "the first of his vigor".

Rabbi Yoḥanan teaches that paternal inheritance does not depend on a spiritual status that begins at conversion, but on biological reality. The father already expended the first of his vigor when he fathered his previous sons as a gentile. While conversion creates a new legal identity, it cannot undo the physical fact of prior fatherhood.

רַבִּי יוֹחָנָן אוֹמֵר: אֵין לוֹ בְּכוֹר לַנַּחֲלָה — דְּהָא הֲוָה לֵיהּ ״רֵאשִׁית אוֹנוֹ״

Rabbi Yoḥanan says: He does not have a firstborn with regard to inheritance, as this man already had 'the first of his strength.' (Bekhorot 47a:2)

Reconciling the Contradictory Baraita

The Gemara challenges Rabbi Yoḥanan from a baraita that explicitly states that a convert who had children as a gentile does have a firstborn for inheritance. Ravina resolves this challenge by demonstrating that the baraita represents the minority position of Rabbi Yosei HaGelili.

Rabbi Yosei HaGelili derives the laws of the father from the laws of the mother. He reads Exodus 13:2 to mean that an opening of the womb must occur "in Israel" to be halakhically recognized. Extending this concept, he argues that prior fatherhood in gentiledom was likewise erased upon conversion.

אָמַר רָבִינָא, וְאִיתֵּימָא רַב אַחָא: הָא וַדַּאי רַבִּי יוֹסֵי הַגְּלִילִי הִוא, דְּאָמַר ״פֶּטֶר רֶחֶם בְּיִשְׂרָאֵל״ — עַד שֶׁיִּפְטְרוּ רֶחֶם מִיִּשְׂרָאֵל, וְיָלֵיף אִיהוּ מִינַּהּ דִּידַהּ.

Ravina says, and some say it is Rav Aḥa who says: This ruling is certainly in accordance with the opinion of Rabbi Yosei HaGelili, who says that the phrase in the verse 'whatever opens the womb among the children of Israel' means it is not a firstborn unless it opens the womb of a woman from the Jewish people. And he derives his halakha, that of a gentile who converted, from her halakha. (Bekhorot 47a:11)

The Halachic Conclusion

The Gemara clarifies that the majority of the sages reject Rabbi Yosei HaGelili's view,. While the mother's firstborn is defined by the sanctification of the womb, the father's firstborn for inheritance is uniquely governed by the physical requirement of "the first of his vigor",.

Halachically, the ruling follows Rabbi Yoḥanan. Because the convert already produced his biological firstborn before entering the covenant, his subsequent son is not entitled to a double portion, and all his post-conversion sons inherit equal shares.

Because the Torah defines the firstborn for inheritance as the father's biological first strength, a convert's prior fatherhood disqualifies his later sons from a double portion.

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