Bechorot 47: Firstborn Child of A Man Who Converts

Are the children born before their parent converted to Judaism recognized as related to him?  Different areas of Jewish law are considered in today's Gemara.  We should note that today's discussion refers to the firstborn child of a man and not necessarily the firstborn child of a woman.  The following is taught:

"If he had children while he was a non-Jew and he converted to Judaism, Rabbi Yochanan says that he cannot have a firstborn who will receive a double portion of the inheritance.  Rabbi Shimon ben Lakish (Reish Lakish) says that he can.  Rabbi Yochanan argues that the father has already had reishit, a beginning; the 'first of his strength' (Devarim 21:17), while Reish Lakish says that he can have a firstborn now with the privilege of inheritance because sh'nitgayer k'katas sh'nolad dami, a stranger who became a convert is like a newly born child."

The Gemara also addresses this disagreement.  It looks at a similar argument between these two rabbis, this time regarding the mitzvah of peru u'revu, propagation (having children).  The following is quoted:

"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 Yochanan says that he has already fulfilled the command of propagation while Reish Lakish disagrees.  Rabbi Yochanan says: He has fulfilled the command, since it is written ' He did not create it to be a wasted, He formed it to be inhabited'.  Reish Lakish says that he has not fulfilled the command of propagation for a stranger who became a proselyte is like a newly born child."

Tosofot explain that Rabbi Yochanan believes that the person who has converted has already fulfilled his obligation in peru u'revu, which is commanded of all people, before he became Jewish.  The Rambam argues that Rabbi Yochanan's ruling can only be applied in a case where the man's children converted together with him, so that he has Jewish children.  The Michat Chinuch suggests that the peru u'revu is unique, in that an action is not simple performed.  Instead, propagation depends upon the outcome - in this case, that would refer to an offspring that is alive.