1) "CHULIN SHE'NA'ASU AL TAHARAS HA'KODESH"
QUESTION: The Gemara questions whether Chulin she'Na'asu Al Taharas ha'Kodesh (food of Chulin which is treated with the sanctity of food of Kodesh) is considered like Kodesh or not. The Gemara attempts to prove from the Mishnah (18b) that it is like Kodesh: if it is not like Kodesh, the Mishnah should list it as an independent category when it lists the various degrees of sanctified items. The Gemara refutes this proof and says that even if Chulin she'Na'asu Al Taharas ha'Kodesh is not like Kodesh, it has the status of either normal Chulin or of Terumah. Since Chulin she'Na'asu Al Taharas ha'Kodesh is included in one of those categories (which the Mishnah already mentions), the Mishnah does not need to list it as a separate category.
RASHI explains that when the Gemara asks that if Chulin she'Na'asu Al Taharas ha'Kodesh is not considered like Kodesh it should be listed by the Mishnah as an independent category, the Gemara refers to the end of the Mishnah. The end of the Mishnah states that the clothing of Perushin (who eat Chulin b'Taharah) is a Midras for those who eat Terumah, and the clothing of those who eat Terumah is a Midras for those who eat Kodesh, and so on. Rashi writes that if Chulin she'Na'asu Al Taharas ha'Kodesh is not like Kodesh, the Mishnah should mention that the clothing of Perushin who eat Chulin b'Taharah ("Al Taharas Chulin") is a Midras for Perushin who eat Chulin she'Na'asu Al Taharas ha'Kodesh.
In contrast, the list of levels in the beginning of the Mishnah is relevant to a different Halachah. One who immerses himself with intent to eat Chulin is permitted to eat Chulin but not Ma'aser. One who immerses with intent to eat Ma'aser is permitted to eat Ma'aser but not Terumah, and so on.
Why does Rashi explain that the proof is from the Mishnah's second list of levels of sanctity, and not from the Mishnah's first list? Rashi should explain that the Gemara's attempted proof is from the list in the beginning of the Mishnah: if Chulin she'Na'asu Al Taharas ha'Kodesh is like Kodesh, then the Mishnah should list it as a separate category and say that one who immerses for Chulin she'Na'asu Al Taharas Chulin is prohibited from eating Chulin she'Na'asu Al Taharas ha'Kodesh, and one who immerses for Chulin she'Na'asu Al Taharas ha'Kodesh is prohibited from eating Ma'aser, and so on. Why does Rashi say that the Gemara's proof is from the list in the end of the Mishnah? (TUREI EVEN)
It makes more sense to explain that the Gemara's proof is from the beginning of the Mishnah because that explanation is more consistent with the words of the Gemara later when it refutes the proof. The Gemara says that there is no difficulty from the words of the Mishnah if Chulin she'Na'asu Al Taharas ha'Kodesh is like ordinary Chulin because the Mishnah already lists Chulin and there is no need to make a new category. Similarly, if Chulin she'Na'asu Al Taharas ha'Kodesh is like Terumah, the Mishnah already lists Terumah. However, only the list in the beginning of the Mishnah mentions Chulin as a separate level. Accordingly, if the Gemara's attempted proof was from the beginning of the Mishnah, the Gemara's refutation of the proof makes sense. The list in the end of the Mishnah makes no mention of Chulin as one of the levels (it merely alludes to it when it says that the clothing of Perushin is a Midras for those who eat Terumah). The Gemara's refutation of the proof implies that the attempted proof was from the beginning of the Mishnah and not from the end! (See MAHARSHA.)
ANSWER: The Gemara earlier (19a) teaches that when one immerses in a Mikvah in order to eat Chulin (b'Taharah), he does not need Kavanah, specific intent. The Gemara questions that assumption from the beginning of the Mishnah which says that "one who immerses for Chulin is permitted to eat Chulin," which implies that he must have specific intent for Chulin. (The Gemara answers that question and concludes that Tevilah for Chulin does not need Kavanah.)
Perhaps Chulin she'Na'asu Al Taharas ha'Kodesh is not like Kodesh, but it also is not entirely like ordinary Chulin in that it does require Kavanah during Tevilah.
Accordingly, no proof may be adduced from the beginning of the Mishnah from the fact that it should have inserted an additional category into its list of levels and stated that if one immerses for Chulin she'Na'asu Al Taharas Chulin, he may not eat Chulin she'Na'asu Al Taharas ha'Kodesh. There would have been no reason for the Mishnah to divide Chulin into two categories, because there is no need for the Mishnah to mention that Tevilah needs Kavanah for Chulin she'Na'asu Al Taharas Chulin (as Tevilah for Chulin needs no Kavanah). When the Mishnah mentions Chulin, perhaps it does refer to Chulin she'Na'asu Al Taharas ha'Kodesh, and thus the Gemara cannot ask why the Mishnah does not mention that category -- it does mention that category!
In contrast, the list in the end of the Mishnah (which discusses the Midras-status of the clothing of those who eat one level of food in relation to those who eat a higher level) could have mentioned both ordinary Chulin and Chulin she'Na'asu Al Taharas ha'Kodesh in the list, because the law of Midras between the levels applies to Chulin she'Na'asu Al Taharas Chulin as well. Accordingly, if Chulin she'Na'asu Al Taharas ha'Kodesh is a separate, independent category, the Mishnah would have added an extra category in its list. (M. Kornfeld)
2) ONE WHO GUARDS AN UNIDENTIFIED OBJECT FROM "TUM'AH"
QUESTION: The Gemara attempts to prove that a person does not need to know what he is guarding in order to successfully guard it from becoming Tamei. The object remains Tahor as long as he guards it from Tum'ah, even though he thinks that the object he is guarding is a different object (he thinks he is guarding one object and it turns out to be a different object).
The Gemara proves this from an incident in which a woman asked Rebbi Yishmael a question about the Taharah of a garment she wove. She said that although she was Tahor while she wove the garment, she did not have in mind to guard it from Tum'ah, but she did not see it come into contact with any source of Tum'ah. Rebbi Yishmael interrogated her and discovered that while she was setting up the loom for the garment, another woman who was a Nidah (and Tamei) helped her, and it is possible that the Nidah moved the loom and rendered the garment Tamei.
Rebbi Yishmael declared, "How great are the words of the Chachamim who said that if one intended to guard an object, it is Tahor, and if one did not intend to guard it, it is Tamei!" The Gemara infers from his declaration that it suffices that one guard the object from Tum'ah and it is not necessary that he know exactly what the object is.
How does the Gemara infer this law from the statement of Rebbi Yishmael? In that incident, the woman knew the identity of the object, but she did not guard it at all. Perhaps Rebbi Yishmael meant that the object is Tahor only when one guards it and knows exactly what he is guarding, but it is not Tahor if one guards it and does not know what he is guarding.
ANSWER: The MITZPEH EISAN answers that the Gemara's proof is from Rebbi Yishmael's affirmation of the wisdom of the Chachamim. If the Halachah requires one to have in mind the particular object which he is guarding, then the Halachah which Rebbi Yishmael quoted must be read, "If one intended to guard this particular object it is Tahor, and if one did not intend to guard this particular object it is Tamei (even though he guarded the object thinking that it was a different object)." Rebbi Yishmael was not saying simply that the object is Tamei if one did not guard it at all, but rather that it is Tamei even if one did guard it but thought that it was a different object.
If this was Rebbi Yishmael's intention, however, how could he affirm the great wisdom of the Chachamim from the incident of the woman whose garment was found to be Tamei? In that case, even if she had guarded it thinking that it was a different object, she still would not have let a Nidah get near it and it would have remained Tahor. The only reason why it was found to be Tamei was because she did not guard it at all, and not because she guarded it without knowing exactly what she was guarding.
It must be that the Chachamim decreed that the guarding is not valid (and the object is Tamei) only when one does not have intent to guard it at all. If he had intent to guard it from Tum'ah in any way (even if he did not know exactly what he was guarding), it suffices and the object remains Tahor.

OTHER D.A.F. RESOURCES
ON THIS DAF