Nazir, Chapter 2, Mishnah 8. The Mishnah returns to the case discussed in the previous Mishnah: a person who accepted nezirus upon himself on condition that he would have a child - "when I have a child." Here the Mishnah deals with a case where his wife miscarried, and there is a doubt whether this miscarriage counts as a child or not.
"Hipilah ishto - eino nazir":
In circumstances of doubt the Mishnah rules "eino nazir" - he is not considered a nazir. The Gemara explains that this is the opinion of Rabbi Yehudah, who holds "a doubt regarding nezirus is treated leniently": in any doubt concerning nezirus we rule leniently. The reason is that the one who vowed did not intend to enter nezirus in a case of doubt, and he did not have in mind when he vowed to accept nezirus out of doubt; his intention was that the vow should take effect only in a case of certainty, when a healthy, viable child is born.
The approach of Rabbi Shimon:
Rabbi Shimon disagrees and holds that one should not ignore the doubt but rather deal with it, and therefore the one who vowed should state a double condition: "im hayah ben kayama - harei ani nezir chovah, ve'im lo - harei ani nezir nedavah":
"Im hayah ben kayama" - if the child that was miscarried was in truth a viable child, one that had the potential to survive, then I am obligated in nezirus by force of the vow that I made.
"Ve'im lo" - and if it was not a viable child, then I did not become obligated in nezirus by force of the vow; nevertheless, "harei ani nezir nedavah" - I accept nezirus upon myself voluntarily.
The result is that either way he will be a nazir: either because he is obligated by force of the vow, or because he accepted voluntary nezirus upon himself.
"Chazrah veyaldah - harei zeh nazir":
Now the Mishnah discusses what happens if, after observing this nezirus, his wife then gave birth to a viable child. According to the first Tanna, who represents the approach of Rabbi Yehudah, "harei zeh nazir": he is now a nazir by force of his original vow, since now the condition of the vow has been fulfilled, whereas the first time it was not fulfilled - for we ruled leniently in the case of doubt.
Rabbi Shimon says: the one who vowed accepted nezirus upon himself because of the possibility that this was a viable child, and since we do not know whether it was a viable child, we also do not know whether he fulfilled his vow. The nezirus he observed was therefore in doubt - perhaps it was the fulfillment of the vow and perhaps it was merely voluntary nezirus. Now, after his wife has given birth to a viable child, either way he is required at some stage to observe an obligatory nezirus and fulfill his vow. Therefore he should say: "im harishon ben kayama - harishon chovah vezeh nedavah, ve'im lo - harishon nedavah vezeh chovah":
"Im harishon ben kayama" - if the first child, the one that was miscarried, was viable, then the first nezirus that I observed was an obligatory nezirus fulfilling my vow, and this one that I am now accepting is a voluntary nezirus - for I am observing it only because of the possibility that the first one already fulfilled the vow.
"Ve'im lo" - and if the first child that was miscarried was not viable, then the first nezirus was voluntary, and this one that I am now observing is an obligatory nezirus, fulfilling the vow that I made at the outset.
In summary: in this Mishnah we learned the dispute between Rabbi Yehudah and Rabbi Shimon regarding a doubt in nezirus. According to Rabbi Yehudah, "a doubt regarding nezirus is treated leniently," and therefore if his wife miscarried he is not a nazir, and when she then gave birth to a viable child he is a nazir by force of his original vow. According to Rabbi Shimon one must deal with the doubt by means of a double condition: in the case of the miscarriage - either an obligatory nazir or a voluntary nazir, and after she then gave birth - either the first was obligatory and this one voluntary, or the first was voluntary and this one obligatory, so that either way the vow will be fulfilled in full.