Mishnah Taharos 4:10: Rabbi Yose says: The uncertainty of [impure] liquids regarding [whether or not they came into contact with] foods is impure [i.e. the purity status of the loaves is uncertain and therefore impure], and regarding vessels it is pure [i.e. when it is uncertain whether or not the liquids came into contact with vessels, the vessels are pure]. How so? If there were two jugs [of water], one impure and the other pure, and someone made dough from [the water from] one of them, and it is uncertain if he made it from the impure one or made it from the pure one, this is the case of the uncertainty of [impure] liquids, regarding foods is impure and regarding vessels it is pure.

רַבִּי יוֹסֵי אוֹמֵר, סְפֵק מַשְׁקִין, לְאֳכָלִים, טָמֵא, וּלְכֵלִים, טָהוֹר. כֵּיצַד. שְׁתֵּי חָבִיּוֹת, אַחַת טְמֵאָה וְאַחַת טְהוֹרָה, וְעָשָׂה עִסָּה מֵאַחַת מֵהֶן, סָפֵק מִן הַטְּמֵאָה עָשָׂה, סָפֵק מִן הַטְּהוֹרָה עָשָׂה, זֶה הוּא סְפֵק מַשְׁקִין לְאֳכָלִין טָמֵא וּלְכֵלִים טָהוֹר:

Mishnah Taharos 4:11: The uncertainty of hands regarding whether they themselves have become impure, or have rendered [something] impure, or have become pure, [in all three cases] they are pure. The uncertainty of the public domain [i.e. any uncertainty regarding purity status which was generated when something was in the public domain], is pure. As for the uncertainty regarding [the purity status of something that came into contact with something impure only according to] the words of the Scholars: If [there is an uncertainty as to whether] one ate impure foods or drank impure liquids, or whether one's head and the majority of one's body came into drawn water, or whether three login of drawn water fell on one's head and a majority of one's body, [all of which are instances of someone whom the Sages decreed becomes impure on a rabbinic level,] in the case of uncertainty, he is pure. But for something which is an Origin of impurity according to the words of the Scholars [i.e. something that the Sages decreed should be treated as an Origin of impurity], in the case of uncertainty [if someone might have come into contact with it], he is impure.

סְפֵק יָדַיִם לִטָּמֵא וּלְטַמֵּא וְלִטָּהֵר, טָהוֹר. סְפֵק רְשׁוּת הָרַבִּים, טָהוֹר. סְפֵק דִּבְרֵי סוֹפְרִים, אָכַל אֳכָלִים טְמֵאִים, שָׁתָה מַשְׁקִים טְמֵאִים, בָּא רֹאשׁוֹ וְרֻבּוֹ בְמַיִם שְׁאוּבִין, אוֹ שֶׁנָּפְלוּ עַל רֹאשׁוֹ וְרֻבּוֹ שְׁלשָׁה לֻגִּין מַיִם שְׁאוּבִים, סְפֵקוֹ טָהוֹר. אֲבָל דָּבָר שֶׁהוּא אַב הַטֻּמְאָה וְהוּא מִדִּבְרֵי סוֹפְרִים, סְפֵקוֹ טָמֵא: