Reference for Tahorot 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:
A vermin and a frog in the public domain, and similarly an olive's volume of a dead body and an olive's volume of an animal carcass, or a bone from a dead body and a bone from an animal carcass, or a clod [of dirt] from a pure land and a clod from a plowed cemetery, a clod from a pure land and a clod from the lands of the nations [i.e. outside Israel], or two paths one impure and one pure, if someone walked down one of them and he does not know which one he walked down, or [in passing by] he covered over one of them [i.e. over the olive's volume of the dead body or of the animal carcass] and he does not know which one he covered over, or he moved one of them [i.e. one of the two bones, or one of the two clods of dirt] and he does not know which one he moved, Rabbi Akiva declares him impure, but the Sages declare him pure.