Commentaire sur Tohorot 5:1
הַשֶּׁרֶץ וְהַצְּפַרְדֵּעַ בִּרְשׁוּת הָרַבִּים, וְכֵן כַּזַּיִת מִן הַמֵּת וְכַזַּיִת מִן הַנְּבֵלָה, וְעֶצֶם מִן הַמֵּת וְעֶצֶם מִן הַנְּבֵלָה, וְגוּשׁ מֵאֶרֶץ טְהוֹרָה וְגוּשׁ מִבֵּית הַפְּרָס, גּוּשׁ מֵאֶרֶץ טְהוֹרָה וְגוּשׁ מֵאֶרֶץ הָעַמִּים, שְׁנֵי שְׁבִילִין, אֶחָד טָמֵא וְאֶחָד טָהוֹר, הָלַךְ בְּאַחַד מֵהֶם וְאֵין יָדוּעַ בְּאֵיזֶה מֵהֶן הָלַךְ, הֶאֱהִיל עַל אַחַד מֵהֶם וְאֵין יָדוּעַ עַל אֵיזֶה מֵהֶן הֶאֱהִיל, הִסִּיט אֶת אַחַד מֵהֶם וְאֵין יָדוּעַ אֵיזֶה מֵהֶם הִסִּיט, רַבִּי עֲקִיבָא מְטַמֵּא, וַחֲכָמִים מְטַהֲרִים:
Une vermine et une grenouille dans le domaine public, et de même un volume d'olive d'un cadavre et un volume d'olive d'une carcasse d'animal, ou un os d'un cadavre et un os d'une carcasse d'animal, ou une motte [de terre] d'une terre pure et une motte d'un cimetière labouré, une motte d'une terre pure et une motte des terres des nations [c'est-à-dire hors d'Israël], ou deux chemins, un impur et un pur, si quelqu'un descendait l'un d'eux et il ne sait pas lequel il a marché, ou [en passant] il a recouvert l'un d'eux [c'est-à-dire le volume d'olive du cadavre ou de la carcasse de l'animal] et il ne sait pas lequel il a recouvert, ou il a déplacé l'un d'eux [c'est-à-dire l'un des deux os, ou l'une des deux mottes de terre] et il ne sait pas lequel il a déplacé, Rabbi Akiva le déclare impur, mais les Sages le déclarent pur.
Bartenura on Mishnah Tahorot
English Explanation of Mishnah Tahorot
And so also [if there was there] an olive's bulk of a corpse and an olive's bulk of carrion,
A bone of a corpse and a bone of carrion;
A clod of clean earth and a clod from a doubtful grave area
A clod of clean earth and a clod from the land of the Gentiles,
Or if there were two paths, the one unclean and the other clean, and a man walked through one of them but it is not known which,
Or if overshadowed one of them but it is not known which, or he shifted one of them but it is not known which:
Rabbi Akiva rules that he is unclean, But the sages rule that he is clean.
Today's mishnah brings up a series of cases in which one of two possible things occurred one which would have caused him to be impure and one in which he was not impure. In the end of the mishnah we can see that the rabbis and Rabbi Akiva disagree as to whether or not the person is pure or impure.
Section one: A dead sheretz causes impurity but a dead frog does not because it is not one of the defiling creepy crawly things listed in Leviticus 11:29.
Section two: Flesh from a corpse defiles in all ways, but flesh from carrion of an animal defiles only by touch and by carrying, not by overshadowing.
Section three: Bone of carrion is pure. Bone from a corpse defiles through contact and carrying.
Sections four and five: Earth from an area that might have been a graveyard defiles through contact and carrying, as does dirt from outside of the land of Israel.
Section seven: This section refers to all of the sections above.
Section eight: Rabbi Akiva says that doubtful cases of impurity in the public domain are pure only if the person didn't perform any action. But in the cases listed here the person moved, carried or overshadowed one of the objects that might have been impure, and therefore he is impure.
The other sages disagree doubtful cases of impurity are always pure in the public domain.