Talmud for Tahorot 4:12
סְפֵק הַחֻלִּין, זוֹ טָהֳרַת פְּרִישׁוּת. סְפֵק שְׁרָצִים, כִּשְׁעַת מְצִיאָתָן. סְפֵק נְגָעִים, בַּתְּחִלָּה טָהוֹר, עַד שֶׁלֹּא נִזְקַק לַטֻּמְאָה. מִשֶּׁנִּזְקַק לַטֻּמְאָה, סְפֵקוֹ טָמֵא. סְפֵק נְזִירוּת, מֻתָּר. סְפֵק בְּכוֹרוֹת, אֶחָד בְּכוֹרֵי אָדָם וְאֶחָד בְּכוֹרֵי בְהֵמָה, בֵּין טְמֵאָה בֵּין טְהוֹרָה, שֶׁהַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה:
The uncertainty [regarding the purity status] of non-sanctified foods, this is the purity of abstention [i.e. of those who are careful to eat in purity even non-sanctified foods; and such foods, when their status is uncertain, are pure]. The uncertainty regarding [something which may or may not have been rendered impure by] vermin, [its status is] in accordance with how they were found [i.e. if the item in question was not actually touching the vermin when it was found, it is assumed to be pure]. The uncertainty of blights [i.e. regarding the purity status of someone that may have been rendered impure by someone impure due to an illness, such as <i>tsara'at</i>], at the beginning he is pure until he is determined [by the priest] as impure; once he has been determined impure, his uncertainty [i.e. a case of someone or something which may or may not have been rendered impure by him] is impure. The uncertainty regarding [one's] status as a nazerite [i.e. if someone took a vow to be a nazerite contingent on something which remains uncertain], he is permitted [to do the things a nazerite is forbidden from doing, i.e. he is not a nazerite]. The uncertainty regarding [the status of] a first born, whether first born humans or first born animals, whether pure or impure [i.e. kosher or non-kosher], the burden of proof is upon the one [who wants] to remove [possessions] from his fellow [i.e. the priest who makes a claim on the first born must demonstrate that it is a first born before he can take it].