בכור ומעשר דחלין על בעל מום קבוע ואין יוצאין לחולין ליגזז וליעבד אלא שם זבח לא קתני
e.g., a firstborn or an animal tithe, the sanctity of which takes effect even on a permanently blemished animal, and this offering cannot vacate its sanctified status and assume non-sacred status for its wool to be sheared and to be worked. Rather, you must say that the baraita is not teaching a general category of sacrifices, but when it states: Offering, it is referring to a particular one.
ומאי שנא תמורה שם תמורה אחת היא זבח איכא בכור ואיכא מעשר
The Gemara asks: And what is different about the two statements, i.e., why does the tanna deal with a specific case in one area, but a general category in the other? The Gemara explains: Substitution is one category, as there is no difference between one case of substitution and another. By contrast, with regard to sacrifices, there is a firstborn and there is the animal tithe, whose halakhot differ from other offerings, and therefore one cannot establish a single general principle. Consequently, the tanna certainly is referring to a specific offering.
ולרב ששת אדמוקים לה באילו של אהרן לוקמה בפסח דדוחה את השבת ואת הטומאה ועושה תמורה דקרבן יחיד הוא קסבר אין שוחטין הפסח על היחיד
The Gemara continues the previous discussion: And according to the opinion of Rav Sheshet, who explains that the offering in question is not the bull of the High Priest but his ram, rather than establishing and interpreting this baraita as referring to the ram of Aaron, let him establish that it deals with the Paschal offering, which overrides Shabbat and ritual impurity and one can perform substitution for it, as according to all opinions, it is the offering of an individual. The Gemara answers: Rav Sheshet maintains that one may not slaughter the Paschal lamb on behalf of an individual, but only for a group. This means that it is not an offering of an individual but, at the very least, that of partners. For this reason, one cannot perform substitution for a Paschal lamb.
ונוקמיה בפסח שני מי דחי טומאה
The Gemara asks: And let Rav Sheshet establish the baraita as referring to the second Pesaḥ, which is slaughtered by an individual. The Gemara answers: Does the second Pesaḥ override ritual impurity? Since this offering does not override ritual impurity, it cannot be the offering referred to in the baraita.
אמר ליה רב הונא בריה דרב יהושע לרבא ותנא מ"ש פסח דקרי ליה קרבן יחיד ומ"ש חגיגה דקרי לה קרבן ציבור אי משום דאתי בכנופיא פסח נמי אתי בכנופיא איכא פסח שני דלא אתי בכנופיא
§ Rav Huna, son of Rav Yehoshua, said to Rava: And according to the tanna of the aforementioned baraita, concerning the dispute between Rabbi Meir and Rabbi Ya’akov, what is different with regard to the Paschal offering, that he calls it the offering of an individual? And what is different with regard to the Festival peace-offering, which is eaten with the Paschal offering, that he calls it a communal offering? If this distinction is because the Festival peace-offering is brought by a multitude, i.e., the entire nation brings it, the Paschal offering is also brought by a multitude, not as an individual offering. Rava replied: There is the second Pesaḥ, which is not brought by a multitude, and therefore the tanna does not call the Paschal offering a communal offering.
אמר ליה אם כן יהא דוחה את השבת ואת הטומאה אמר ליה אין כמאן דאמר דחי דתניא פסח שני דוחה את השבת ואינו דוחה את הטומאה ר' יהודה אומר אף דוחה את הטומאה מ"ט דתנא קמא אמר לך מפני טומאה דחיתו ויעשה בטומאה
He said to him: If so, that the second Pesaḥ is a communal offering, it should override Shabbat and ritual impurity. He said to him: Yes, as the opinion of this tanna is in accordance with the one who said that the second Pesaḥ overrides ritual impurity. As it was taught in a baraita: The second Pesaḥ overrides Shabbat, but it does not override ritual impurity. Rabbi Yehuda says: It even overrides ritual impurity. The Gemara asks: What is the reason of the first tanna? The first tanna could have said to you that one brings a second Pesaḥ solely because ritual impurity overrode his obligation to sacrifice the first Pesaḥ, i.e., he did not sacrifice the first Pesaḥ because he was impure at that time. And should he now perform the second Pesaḥ in a state of ritual impurity?
ורבי יהודה אמר לך אמר קרא (במדבר ט, יב) ככל חקת הפסח יעשו אותו ואפילו בטומאה התורה החזירה עליו לעשותו בטהרה לא זכה יעשנו בטומאה
And Rabbi Yehuda could have said to you that, with regard to the second Pesaḥ, the verse states: “According to all the statute of the Paschal offering they shall keep it” (Numbers 9:12), which indicates that it should even be brought in a state of ritual impurity, unlike the first Pesaḥ. As for the claim of the first tanna, that the whole reason for the second Pesaḥ is due to ritual impurity, Rabbi Yehuda could respond: The Torah sought an opportunity for one who was impure at the time of the first Pesaḥ to perform it in a state of ritual purity; if he did not merit to perform it in purity, he should nevertheless perform it even in a state of ritual impurity.