Gemara

32b (Mishnah): Six men (Reuven, Shimon...) saw a man walking towards them.

Reuven: I am a Nazir if the man approaching us is Ploni.

Shimon: I am a Nazir if it is not Ploni.

Levi: I am a Nazir if one of you (Reuven and Shimon) is a Nazir,'

Yehudah: I am a Nazir if one of you (Reuven and Shimon) is not a Nazir,'

Yisachar: I am a Nazir if both of you are Nezirim.

Zevulun: I am a Nazir if all five of you are Nezirim.

Beis Shamai say, all six of them are Nezirim.

Beis Hillel say, only those whose words were not fulfilled are Nezirim;

R. Tarfon says, none of them are Nezirim.

R. Shimon says, each says 'if I was correct, I am a Nazir from my previous acceptance. If I was wrong, I now accept Nezirus on myself.'

Question: Why do Beis Hillel say that those words were not fulfilled are Nezirim?!

Answer #1 (Rav Yehudah): The Mishnah should say 'those whose words were fulfilled.'

Answer #2 (Abaye): The case is, he said 'even if it is not Ploni, I will be a Nazir.' The Mishnah means, even one whose first words were not fulfilled, but his latter words were fulfilled (is a Nazir).

33a (Mishnah): If the approaching man reversed direction (and we do not know whether or not it was Ploni), he (each of the first two men) is not a Nazir;

Inference: Had we recognized him, he (who was correct) would be a Nazir!

Question: Like whom is our Mishnah? It is unlike R. Tarfon. Since when he vowed, he did not know whether or not it was Ploni, he would not be a Nazir!

(Beraisa - R. Yehudah citing R. Tarfon): None of them are Nezirim, because Nezirus must be accepted with certainty.

Answer: The Mishnah is like R. Yehudah's own opinion in the Beraisa of the stack (that one accepts Nezirus only if his words will be verified):

(Beraisa - R. Shimon): If one said 'I am a Nazir on condition that this stack contains 100 Kor (a measure)', and he found that it was stolen and he could not measure it, he is forbidden (all Isurei Nezirus);

R. Yehudah permits.

R. Shimon holds that had we been able to find that it holds 100 Kor, he would be a Nazir. Even now that it was stolen, he is a Nazir (due to the Safek);

Also in our Mishnah, R. Shimon would say that had we verified that it is Ploni, Reuven would be a Nazir, so also when we are left in a Safek, he is a Nazir!

Nedarim 19a - Contradiction: Elsewhere, R. Yehudah holds that a person forbids himself through Safek!

19b - Answer (Rava): R. Yehudah holds that a person does not enter a Safek that is more stringent than Vadai (Nezirus). A Vadai Nazir can shave (at the end of Nezirus) and bring Korbanos, and the Korbanos are eaten. A Safek Nazir cannot!

Rishonim

Rambam (Hilchos Nezirus 2:8): David said 'the man approaching is Shimon.' Levi said 'it is Reuven.' David said 'I am a Nazir if he will be Reuven', and Levi said 'I am a Nazir if he will be Shimon.' If it is Reuven, David is a Nazir. If it is Shimon, Levi is a Nazir.

Kesef Mishneh: This is like Beis Hillel, according to Rav Yehudah.

Question (Mishneh l'Melech): Tosfos explains that David (and Levi) accepted Nezirus 'if I am right (that this is Reuven/Shimon).' (Our Tosfos does not say so explicitly.) This forced Rav Yehudah to change the text of Beis Hillel to say 'those whose words were fulfilled' are Nezirim. The Rambam explains that David and Levi each accepted Nezirus 'if I am wrong', like the Mishnah's text of Beis Hillel, 'those whose words were not fulfilled are Nezirim.' Even though there is no difference in Halachah, why did he explain unlike the Gemara?!

Answer (Tosfos Yom Tov 5:1 DH u'Veis): One may explain the Mishnah unlike the Gemara if it does not change the Halachah. Likewise, there is free reign to explain verses, as long as it does not oppose laws from the Gemara.

Note: The Mishneh l'Melech below shows that there is a Halachic difference.

Maharit (1:19 DH v'Al): The Mordechai (Sof Bava Metzia) exempts one who vowed to give a certain sum to Tzedakah if he will jest (and he jested), for this is Asmachta (a commitment that one did not expect to need to fulfill). We learn from R. Tarfon. He says that none of them are Nezirim, due to Asmachta. Beis Hillel argue only when he intended for Nezirus and his words were fulfilled. All agree that one does not become a Nazir through Asmachta. This is difficult. Nedarim 19b establishes the Beraisa of a stack to be Asmachta. R. Yehudah exempts only because Safek Nezirus is more stringent than Vadai. The Mordechai holds that all exempt due to Asmachta, regardless of the stringency of Safek! Rather, only R. Tarfon exempts. Why did the Rambam establish our Mishnah to be Asmachta (he accepted Nezirus if he errs)? The Mishnah connotes otherwise. Why would Beis Shamai obligate even one whose words were not fulfilled?! This is not even mistaken Nezirus, for he never intended to be a Nazir! The Rambam wasn't concerned for this. He merely wanted to rule like Beis Hillel, that even in a case of pure Asmachta, if his words were not fulfilled (he was wrong) he is a Nazir.

Rebuttal (Mishneh l'Melech): The Rambam holds that our Mishnah truly discusses Asmachta, like he establishes it in Perush ha'Mishnayos! Rather, he rules like Abaye, but he had a different text. Our text is difficult. If he said 'and even if it is not Ploni, I am a Nazir', obviously in any case he is a Nazir! Why does R. Tarfon say that none of them are Nezirim? Perhaps he said only 'and if not', and this helps only Toch Kedei Dibur of his first acceptance. This is difficult. Rather, the Rambam holds that Rav Yehudah explained the Mishnah simply (he accepted Nezirus if he is correct). This forced Rav Yehudah to change the text of Beis Hillel. Abaye explains that first he said 'this is not Ploni.' Later, he said 'I am a Nazir if it is Ploni.' He is a Nazir if his initial words were wrong. Rav Yehudah did not want to explain this way, for he holds that Beis Hillel agree with R. Tarfon that one who vows through Asmachta is not a Nazir, because he did not intend to be a Nazir.

Maharit (ibid.): Regarding 'this cow said 'I will be a Nazir if I will get up'' (Nazir 10a), all agree that Nezirus takes effect even through Asmachta. If one said 'I will be a Nazir if I will not reveal (Pasul) families', he should be a Nazir and not reveal. Nedarim take effect through Asmachta in many cases. This shows that Beis Hillel do not expound "Ki Yafli" to exclude Asmachta. In Nedarim 21a, we equate Hafla'ah of Nezirus and of Nedarim. The Rashba (Teshuvah 183 attributed to Ramban) learns from Nezirus that Asmachta to Hekdesh acquires. R. Tarfon was after the Bas Kol taught that the Halachah follows Beis Hillel. He does not exempt due to Asmachta. Rather, he expounds that Nezirus must be explicit at the time it takes effect. The Ramban says that Asmachta applies only to monetary matters.

Rambam (ibid.): The same applies to all similar cases. If the approaching man reversed direction and we do not know who it was, neither of them is a Nazir.

Rambam (9): Similarly, if one said 'I am a Nazir on condition that this stack contains 100 Kor', and went to measure it and found that it was stolen or lost and he could not measure it, he is not a Nazir, for we are lenient about Safek Nezirus.

Radvaz: The Halachah follows R. Yehudah against R. Shimon. Also, the Stam Mishnah is like R. Yehudah. We are lenient about every Safek Nezirus.