Nazir, Chapter 2, Mishnah 6. Our Mishnah deals with a subject similar to one we have encountered before: is a person's initial declaration the one that determines the halachah, or are his later words also taken as an explanation of his opening words?
The text of the Mishnah and its explanation:
"Harei alai legaleiach chatzi nazir" - I take upon myself to shave half a nazir. As we mentioned in the previous Mishnah, a person accepts upon himself to bring the offerings of a nazir; here, though, he accepts upon himself the offerings of half a nazir. In reality there is no such thing as "half a nazir" - nezirus is a complete nezirus.
"V'shama chaveiro v'amar: 'Va'ani alai legaleiach chatzi nazir'" - and his friend heard him and said: "And I too take upon myself to shave half a nazir." This detail is not relevant to the actual discussion in our Mishnah; it is a continuation of the previous Mishnah, and it adds nothing to the case.
The opinion of Rabbi Meir:
"Zeh megaleiach nazir shaleim v'zeh megaleiach nazir shaleim" - this one shaves a complete nazir and this one shaves a complete nazir. Each of them is obligated to supply the offerings of a full nazir, because saying "half a nazir" is regarded as nothing more than a retraction of what he set out to say at first. When he said "harei alai legaleiach," the understanding is that he accepted upon himself all the offerings of a nazir, and by adding the word "half" he accomplishes nothing. This is the principle of "tafas lashon rishon" - we accept the consequences of a person's first words, and the fact that he said "half a nazir," something that does not exist in reality, has no effect at all.
The opinion of the Sages:
"Zeh megaleiach chatzi nazir v'zeh megaleiach chatzi nazir" - this one shaves half a nazir and this one shaves half a nazir. Each of them supplies half the offerings of a nazir, because the Sages hold that this is a case of "neder u'fischo imo" - a vow whose opening is drawn along with it, meaning that the interpretation is contained within the vow itself. Since he concluded his words with "megaleiach chatzi nazir," his intention is to say that he shaves half of a nazir and supplies half of his offerings, almost as though he had said: "Half the offerings of a nazir are upon me."
In summary: in this Mishnah, Rabbi Meir and the Sages disagree about a person who said "harei alai legaleiach chatzi nazir": according to Rabbi Meir we apply "tafas lashon rishon," and therefore his initial acceptance stands in force and he brings the offerings of a complete nazir; according to the Sages this is a case of "neder u'fischo imo," and the end of his words explains their beginning, and therefore he brings only half the offerings of a nazir.