(Mishnah - R. Eliezer): "Sheretz Tamei v'Nelam" teaches that he brings a Korban for Helam (forgetting the) Sheretz (Tum'ah), but not for Helam Mikdash;

R. Akiva says, "v'Nelam Mimenu v'Hu Tamei" - he brings a Korban for Helam Tum'ah, but not for Helam Mikdash. (The Gemara will discuss whether he argues with R. Eliezer.)

R. Yishmael says, it says "v'Nelam Mimenu" twice, to obligate for Helam Tum'ah and for Helam Mikdash.

(Gemara - Chizkiyah): [R. Eliezer and R. Akiva] argue. (R. Eliezer requires that he knew how he became Tamei. R. Akiva does not.)

(R. Yochanan): They argue only about how to expound. (No one requires that he knew how he became Tamei.)

Some (R. Akiva) teach that he is liable for Helam Tum'ah, but not for Helam Mikdash, and some (R. Eliezer) teach that he is liable for Helam Sheretz, but not for Helam Mikdash. (They do not argue about the Halachah.)

Question: [According to Chizkiyah] R. Eliezer contradicts himself! There (Kerisus 4:2) he said that even if one does not know [whether he ate Chelev or Nosar, he bring Chatas Kavu'a], and here he says that he must know [which Tum'ah]!

Answer: There, "v'Hu Tamei" teaches that he must know how he became Tamei. Here, "Asher Chata Bah" - in any case.

Question: R. Yehoshua contradicts himself! (We assume that he holds like R. Akiva, that he need not know which Tum'ah. In Kerisus, he requires knowing how he became obligated Chatas Kavu'a)!

Answer: There, it says "v'Hu Tamei" - in any case. Here it says "Asher Chata Bah" - he must know for what he is obligated.

Question (R. Chinena): It is not written here (Chatas Kavu'a) "Bah"?! How does R. Eliezer expound "Bah"? (He should agree with R. Yehoshua!)

Answer: It excludes one who was Mis'asek (engaged in another matter, e.g. on Shabbos he was trying to cut something detached, and he cut something attached).