My notes say that once Abel was killed, he lost his fulfillment of the mitzvah of "be fruitful and multiply." Cain was not considered a son of Adam in this regard (Pirkei D' Rebbi Eliezer; Yevamos 62a).
1. Once a person has fulfilled the "multiply" mitzvah, he can't unfulfill it due to his kid's action, can he?
2. He still has Cain. Why doesn't he count? If one's son murders another, is the father penalized re that mitzvah?
Barry Epstein, Dallas, USA
The "multiply" Mitzvah is actually constant, but one is exempted from actively fulfilling it as long as one has two living children. If one child dies (without leaving behind children of his own) the requirement remains. The father has not "unfulfilled" the Mitzvah, he merely lost his exemption. (That is, he still retains the reward for the time during which the Mitzvah was fulfilled).
The same is true with regard to Cain. Having borne him was considered the fulfillment of a Mitzvah, until he sinned. But afterwards, it is considered as if he had died.
I was under the impression that the "multiply" mitzvah or "pru u'ravu" was only fulfilled when both a female and male offspring were produced and then only when each of them proved capable of having female and male offspring. Secondly, I believe that what you are describing in your answer may apply more readily to the mitzvah of "sheves" or "populating" rather than "mulipyling." Please elaborate.
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Yes, that is correct. When both a female and male offspring were produced and each of them proved capable of having offspring, the father is no longer required to have children. If the offspring die, though, the Gemara (Yevamos 62a) records a Machlokes among the Amora'im whether the father must have another two children. The conclusion is that he must. This interpretation of the Mitzvah of being fruitful ("Pru u'Revu") is in fact inferred from the verse "Lasheves Yetzarah", to which you referred (Yevamos, ibid.).
By the way, there indeed is a technical problem with the Gemara that Barry Epstein referred us to (which records an opinion that Adam had to have another child to make up for Hevel, after Hevel was killed, since 2 boys and 2 girls are required by Halachah). As the Mahadura Basra points out, *Rav Huna*, who says Adam had to have a replacement for Hevel, is the same Amora who maintains that when a child dies it is *not* necessary to have a "replacement" child!
Also, Barry mentioned a Pirkei d'Rebbi Eliezer who says that *Cain* was no longer considered a son of Adam after the point at which he killed his brother. I found a Pirkei d'Rebbi Eliezer (#21) that says Cain was not like Adam's son and did not follow Adam's or Hevel's ways, but it does not seem to be dealing with the issue of Adam's Mitzvah of being fruitful. In fact, to the contrary - from the Gemara in Yevamos that we are discussing it seems clear that *only* Hevel had to be replaced by Adam in order to fulfill his Mitzvah, not Cain.