Before us is Mishnah 4 in the second chapter of Maseches Nazir, which deals with a person who accepts nezirus upon himself conditionally, or out of a mistake and a faulty understanding of the laws of nezirus. We will study three cases, and in each one we will examine whether the nezirus takes effect.
"Hareini nazir al menas she'ehei shoseh yayin u'mitamei la'meisim":
A person wishes to accept nezirus upon himself, but on condition: that he be permitted to drink wine or to become tamei to the dead - two things the Torah forbade to a nazir. The Mishnah rules: "Harei zeh nazir, ve'asur be'kulan" - he is a nazir in every respect, and he is bound by all the prohibitions without any condition.
The reason is the well known rule: whoever stipulates a condition contrary to what is written in the Torah - his condition is void. Since the condition he made contradicts Torah law, the condition is null and void, and the vow of nezirus stands in full force.
"Yodea ani she'yesh nezirus ve'eini yodea she'hanazir asur be'yayin":
Here the person is not making a condition; rather, he is mistaken about one detail: he knows that there is such a thing as nezirus and that it includes various prohibitions, but he did not know that a nazir is forbidden to drink wine. According to the Tanna Kamma, "harei zeh asur" - the nezirus takes effect, and he is bound by all the laws of nezirus, wine included. But "Rabbi Shimon matir" - Rabbi Shimon permits.
What is the root of the dispute?
According to the Tanna Kamma: from the moment a person accepts upon himself any part of nezirus, even a single aspect of it, he is considered a nazir and the entire matter applies to him.
According to Rabbi Shimon: a person is not considered a nazir unless he accepts upon himself all the aspects of nezirus. If he did not accept them all, the nezirus itself does not take effect.
Accordingly, this person, who thought there is a nezirus that permits drinking wine - not because he stipulated a condition, but out of a lack of knowledge in that part of the halachah - did not accept upon himself all the parts of nezirus, and therefore, according to Rabbi Shimon, his nezirus does not take effect.
"Yodea ani she'hanazir asur be'yayin aval savur hayisi she'chachamim matirim li":
In the third case the person took a vow of nezirus, and afterward claimed: I know all the laws of nezirus well, including that a nazir is forbidden wine, but I thought that the Chachamim would permit it to me and make an exception for me. And how so? In two ways:
Regarding wine: "mipnei she'eini yachol lichyos ela be'yayin" - since I cannot live without wine, I thought they would permit it to me for that reason.
Regarding tumah: since I earn my living burying the dead and digging graves, I thought they would surely permit me to become tamei to a corpse.
The Tanna Kamma says: "harei zeh mutar" - he is not a nazir, since his nezirus came about through a mistake. This case falls into the category we learned in the third chapter of Maseches Nedarim, where four types of vows that are permitted were listed, among them nidrei onsim and nidrei shegagos - vows a person makes out of a mistaken understanding of some aspect of the vow, and the vow is not valid.
Rabbi Shimon disagrees and says that in this case the vow is binding, meaning: binding until a chacham or a beis din annuls it. He does not mean that there is no remedy for the vow - it can be annulled, but it requires annulment. For Rabbi Shimon holds that those four vows mentioned in Maseches Nedarim are not automatically permitted; rather, they can and must be annulled by a chacham or a beis din.
In summary: in this Mishnah we learned three cases. One who accepts nezirus on condition that he may drink wine and become tamei to the dead - the condition is void under the rule of stipulating against what is written in the Torah, and he is a nazir bound by all the prohibitions. One who did not know that a nazir is forbidden wine - the Tanna Kamma holds that accepting part of nezirus obligates one in all of it, while Rabbi Shimon holds that there is no nezirus unless all its aspects are accepted. And one who knew the law and thought the Chachamim would permit it to him - according to the Tanna Kamma this is a mistaken vow and is permitted on its own, while according to Rabbi Shimon it is binding until a chacham or a beis din annuls it.