Tosafot explain that according to Rabbi Yohanan, 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 Yohanan’s ruling is true only in a case where the man’s children converted together with him, so he has Jewish children. The Minhat Hinukh 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 mitzvah is dependant on the outcome. If the man has living offspring, he has fulfilled the commandment.
The Gemara on today’s daf (=page) asks whether the children of a convert who were born while he was still a non-Jew are recognized by halakhah 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 Yohanan 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 Yohanan holds that he cannot have a firstborn with respect to inheritance, for he already had reishit ono ‘the beginning 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 Yohanan and Rabbi Shimon ben Lakish with regard to the mitzvah 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 Yohanan says: He has already fulfilled the command of propagation, whereas Rabbi Shimon ben Lakish says: He has not fulfilled the command. Rabbi Yohanan says: He has fulfilled the command, since it is written, “He God hath created it not in vain, 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.