פירוש על שבועות 2:5

רבי אליעזר אומר השרץ ונעלם – as it states in Scripture (Leviticus 5:2): “or the carcass of an unclean creeping thing – and the fact has escaped him,” it is an additional Biblical verse, for above, it is written, “or when a person touches any unclean thing” (Leviticus 5:2), and the creeping thing is included in the unclean thing, but rather, to teach you that if he knew that he had certainly been defiled, but he doesn’t know if it was a creeping thing or if it was through a carcass, he is not liable for a sacrifice if he ate Holy Things until he has awareness at the beginning if it was through a creeping thing that he was defiled or through a carcass that he was defiled. But Rabbi Akiva holds that since he definitely knew that he was defiled, even though it was not made clear to him if the defilement that he sustained was with a creeping thing or a carcass, he is liable. And both of them exempt him from unawareness of the Temple (i.e., that he entered the Temple while ritually impure, aware that he was ritually impure, but unaware that he has entered the Temple. He is liable for a sin-offering as atonement). Rabbi Yishmael requires [the sacrifice] and the Halakha is according to Rabbi Yishmael.

Rabbi Eliezer said: “[Scripture says: ‘If any one touch… the carcass of] an impure creeping thing, and it be unknown to him’ (Leviticus 5:2), when the impure creeping thing is unknown to him, he is liable; but he is not liable, when the [fact that he is in the] Temple is unknown to him.”
Rabbi Akiba said: “[Scripture says:] ‘and it be hidden from him that he is impure’: when his impurity is unknown to him, he is liable; but he is not liable, when the [fact that he is in the] Temple is unknown to him.”
Rabbi Ishmael said: “[Scripture] twice [says:] ‘and it be hidden from him’, in order to make him liable both for the forgetfulness of the impurity and the forgetfulness of the Temple.”
Mishnah five conclude the discussion of a person who enters the Temple while not realizing that he is impure. It contains a midrashic discussion which took place between Rabbis Eliezer, Akiba, and Ishmael.
As we have explained throughout this chapter, in order to be obligated to bring a sliding scale sacrifice one must have known something and then forgotten it and then remembered it later on. In both mishnah one and mishnah three of this chapter we assumed that what was known and forgotten was either the fact that he was impure or the fact that he was in the Temple or both. This is the opinion of Rabbi Yishmael in section three. His opinion is the dominant opinion in this chapter of mishnah.
Rabbi Eliezer holds that what was known and then forgotten was specifically the source of the impurity, be it an impure creeping thing or other source of impurity mentioned in Leviticus 5:2 (carcasses of pure animals). If he forgot that he was in the Temple he is not obligated to bring a sliding scale sacrifice. Likewise if never knew that he had been made impure by an impure creepy thing, but merely knew that he was impure, he is not obligated.
Rabbi Akiva holds that what was forgotten was the fact of impurity but not the fact that he was in the Temple. Unlike Rabbi Eliezer, Rabbi Akiva does not limit the knowledge and subsequent forgetfulness to the source of the impurity. Even if he never knew what the source of the impurity was, but knew only that he was impure and then forgot, he will be obligated to bring a sliding scale sacrifice. Rabbi Akiva disagrees with Rabbi Yishmael in that the latter holds that being aware and then forgetting that one was in the Temple also makes him obligated for bringing a sliding scale sacrifice, whereas Rabbi Akiva (and Rabbi Eliezer) believe that it does not.