Nazir, Chapter 5, Mishnah 5. In this mishnah we take up the law of a nezirus accepted conditionally, where the condition depends on clarifying a fact that is not known at the time the vow is made. We will examine the case itself, the dispute between Beis Shammai and Beis Hillel, and the view of Rabbi Tarfon.
The case in the mishnah:
"Hayu mehalchin baderech ve'echad ba kenegdam" - two men were walking along the road, and one man was coming toward them. Each of the travelers accepts nezirus upon himself on a condition relating to the identity of the approaching man:
"Hareini nazir shezeh peloni" - the first says: I accept nezirus upon myself on condition that this is so and so. He believes that this is that man, and if it is, he wishes to be a nazir.
"Hareini nazir she'ein zeh peloni" - the second says: I will be a nazir if this is not that man.
And there are those who accept nezirus upon themselves on a condition that depends on the statements of the first two:
"Hareini nazir she'echad mikem nazir" - a third man who is with them says: I am a nazir on condition that one of you is a nazir. This condition will certainly be fulfilled, since only one of the two is correct in what he said.
"She'ein echad mikem nazir" - he wishes to become a nazir on condition that one of them is not a nazir. This condition too must necessarily be fulfilled, since one of them is mistaken.
"Sheshneichem nezirim" - he vows that both of them be considered nezirim.
"Shekulchem nezirim" - he vows that all of them are nezirim.
The dispute between Beis Shammai and Beis Hillel:
Beis Shammai say: "Kulam nezirim" - they are all nezirim, since every one of them said "Hareini nazir." Even if he was mistaken and was not precise in what he said, according to Beis Shammai he is a nazir, for a nezirus made in error is a valid nezirus, just as property consecrated in error is nonetheless consecrated.
And Beis Hillel say: "Ein nazir ela mi shelo niskaymu devarav" - not all of them are nezirim, and the only nazir here is the one whose words were not fulfilled. The Gemara emends the wording and establishes that it should read: "Ein nazir ela mi sheniskaymu devarav" - the only nazir is the one whose words were fulfilled, that is, one who said "I will be a nazir if such and such is true" and it turns out that he spoke the truth. But if the matter was in error, he is not a nazir.
The view of Rabbi Tarfon:
"Rabbi Tarfon omer: ein echad mehem nazir" - in all of these cases none of them is a nazir. Why? Rabbi Tarfon derives from a verse that the very nature of nezirus requires that it be clear at the moment the vow is accepted whether the person is a nazir or not. In all of these cases the matter is not clear, since the identity of the man coming toward them has not yet been revealed, and therefore the nezirus does not take effect.
In summary: in this mishnah we learned the law of a nezirus that depends on a condition which is not clarified at the time of the vow: according to Beis Shammai they are all nezirim, since a nezirus made in error is a valid nezirus, like property consecrated in error; according to Beis Hillel, the only nazir is the one whose words were fulfilled (according to the Gemara's version of the text); and according to Rabbi Tarfon none of them is a nazir, because the nezirus must be clear at the moment the vow is accepted.