Nazir chapter 2, mishnah 1. The mishnah opens with an intriguing case: a person says "Hareini nazir min ha-grogros u-min ha-develah" - "I am hereby a nazir from dried figs and from pressed figs." He wants to take a vow of nezirus upon himself, but instead of mentioning wine - the thing a nazir is forbidden in - he attaches his nezirus to figs: 'grogros' are dried figs, and 'develah' is moist figs pressed together into a round cake. In effect he has created a 'nezirus from figs,' which does not exist at all. The question is how to interpret such a statement.
The dispute between Beis Shammai and Beis Hillel:
The mishnah brings two opinions: "Beis Shammai omrim nazir, u-Beis Hillel omrim eino nazir" - Beis Shammai say he is a nazir, and Beis Hillel say he is not a nazir. The basis of the dispute is as follows:
The view of Beis Shammai: "Ein adam motzi devarav le-vatalah" - a person does not utter words for nothing. Therefore, the moment he said "Hareini nazir," the nezirus took effect and he became an ordinary nazir in every respect, forbidden in wine and in the rest of the nazir's prohibitions. The added words "min ha-grogros u-min ha-develah" are nothing but an attempt to retract what he has already said, and since the first statement was valid, "he consecrated and then tried to retract," and there is no retracting. Something that carries sanctity - and nezirus is included in that, since it says of a nazir "kadosh yihyeh" - "he shall be holy" - once it has been declared, there is no room to take it back. It emerges that already before he reached the words "min ha-grogros u-min ha-develah" he was a nazir, and mentioning the figs accomplishes nothing.
The view of Beis Hillel: "Bi-gmar devarav adam nispas" - a person is held to the end of his words. Such a statement is considered "a vow with its own opening," that is, a vow whose grounds for annulment are contained within it: the person himself explains in his words what he meant - nezirus from dried and pressed figs, which does not exist - and therefore the nezirus does not take effect at all.
The underlying principle in the dispute:
It is worth pausing on the fundamental point they are debating: when a person speaks, do we seize upon the first part of his words as a statement standing on its own, or do we wait to hear the entire sentence and only then judge its overall meaning? This is a principle that recurs in many areas of halachah.
The words of Rabbi Yehudah:
Rabbi Yehudah presents a new angle in understanding the position of Beis Shammai: "Af ke-she-amru Beis Shammai, lo amru ela be-omer harei hen alai korban" - even when Beis Shammai said their ruling, they said it only about one who says "they are upon me like a korban." That is, Beis Shammai's opinion that the declaration takes effect was stated only in a case where the one vowing says that his intention was to forbid the dried and pressed figs to himself like a korban, by way of a vow, and not to accept nezirus upon himself; the language of nezirus was merely a manner of speech. In such a case Beis Shammai say the declaration takes effect, since a person can indeed forbid these figs to himself with a vow, while Beis Hillel say that even in this case the vow does not take effect.
In summary: in this mishnah we learned the law of one who says "I am hereby a nazir from dried figs and from pressed figs" - a nezirus that does not exist. Beis Shammai obligate him in full nezirus, because "a person does not utter words for nothing" and in a matter of sanctity there is no retracting; Beis Hillel exempt him, because "a person is held to the end of his words," and this is like a vow whose grounds for annulment were stated together with it. Rabbi Yehudah limits the view of Beis Shammai to one who says "they are upon me like a korban." And at the depth of the dispute lies the general question of whether a person's words are measured by how they begin or by how they end.