The Gemara on today’s daf asks whether the children of a convert who were born while he was still a non-Jew are recognized by halakha as being related to him with regard to different areas of Jewish law.
The Gemara teaches:
If he had children while he was a non-Jew and he converted to Judaism, Rabbi Yoḥanan says: He cannot have a firstborn who will receive a double portion of the inheritance, whereas Rabbi Shimon ben Lakish says: He can have a firstborn with respect to inheritance. Rabbi Yoḥanan holds that he cannot have a firstborn with respect to inheritance, for he already had reishit ono ‘the first of his strength’ (see 21:17), whereas Rabbi Shimon ben Lakish says that he can have a firstborn now with the privilege of inheritance, because ger she-nitgayer ke-katan she-nolad dami – a stranger who became a convert is like a newly-born child.
In explanation of this disagreement, the Gemara points to a parallel argument between Rabbi Yoḥanan and Rabbi Shimon ben Lakish with regard to the mitzva of peru u’revu – propagation.
And they both follow their own line of reasoning elsewhere. For it has been stated: If he had children while he was a non-Jew and he converted, Rabbi Yoḥanan says: He has already fulfilled the command of propagation, whereas Rabbi Shimon ben Lakish says: He has not fulfilled the command. Rabbi Yoḥanan says: He has fulfilled the command, since it is written, “He did not create it to be a waste, He formed it to be inhabited,” whereas Rabbi Shimon ben Lakish says: He has not fulfilled the command of propagation, for a stranger who became a proselyte is like a newly-born child.
Tosafot explain that according to Rabbi Yoḥanan, the convert already fulfilled his obligation in peru u’revu – a commandment to all mankind – before he became Jewish. The Rambam disagrees and argues that Rabbi Yoḥanan’s ruling is true only in a case where the man’s children converted together with him, so he has Jewish children. The Minḥat Ḥinukh explains that the commandment of peru u’revu is unique. Unlike other mitzvot where the point is the action that is performed, fulfillment of this mitzva is dependent on the outcome. If the man has living offspring, he has fulfilled the commandment.