Reference for Makhshirin 6:8
חֲלֵב הָאִשָּׁה מְטַמֵּא לְרָצוֹן וְשֶׁלֹּא לְרָצוֹן, וַחֲלֵב הַבְּהֵמָה אֵינוֹ מְטַמֵּא אֶלָּא לְרָצוֹן. אָמַר רַבִּי עֲקִיבָא, קַל וָחֹמֶר הַדְּבָרִים. מָה אִם חֲלֵב הָאִשָּׁה שֶׁאֵינוֹ מְיֻחָד אֶלָּא לִקְטַנִּים, מְטַמֵּא לְרָצוֹן וְשֶׁלֹּא לְרָצוֹן, חֲלֵב הַבְּהֵמָה שֶׁהוּא מְיֻחָד לִקְטַנִּים וְלִגְדוֹלִים, אֵינוֹ דִין שֶׁיְּטַמֵּא לְרָצוֹן וְשֶׁלֹּא לְרָצוֹן. אָמְרוּ לוֹ, לֹא, אִם טִמֵּא חֲלֵב הָאִשָּׁה שֶׁלֹּא לְרָצוֹן, שֶׁדַּם מַגֵּפָתָהּ טָמֵא, יְטַמֵּא חֲלֵב הַבְּהֵמָה שֶׁלֹּא לְרָצוֹן, שֶׁדַּם מַגֵּפָתָהּ טָהוֹר. אָמַר לָהֶם, מַחְמִיר אֲנִי בְּחָלָב מִבְּדָם, שֶׁהַחוֹלֵב לִרְפוּאָה, טָמֵא, וְהַמַּקִּיז לִרְפוּאָה, טָהוֹר. אָמְרוּ לוֹ, סַלֵּי זֵיתִים וַעֲנָבִים יוֹכִיחוּ, שֶׁהַמַּשְׁקִים הַיּוֹצְאִין מֵהֶן לְרָצוֹן, טְמֵאִים, וְשֶׁלֹּא לְרָצוֹן, טְהוֹרִים. אָמַר לָהֶן, לֹא, אִם אֲמַרְתֶּם בְּסַלֵּי זֵיתִים וַעֲנָבִים, שֶׁתְּחִלָּתָן אֹכֶל וְסוֹפָן מַשְׁקֶה, תֹּאמְרוּ בְחָלָב שֶׁתְּחִלָּתוֹ וְסוֹפוֹ מַשְׁקֶה. עַד כָּאן הָיְתָה תְשׁוּבָה. אָמַר רַבִּי שִׁמְעוֹן מִכָּאן וָאֵילָךְ הָיִינוּ מְשִׁיבִין לְפָנָיו, מֵי גְשָׁמִים יוֹכִיחוּ, שֶׁתְּחִלָּתָן וְסוֹפָן מַשְׁקֶה וְאֵינָן מְטַמְּאִין אֶלָּא לְרָצוֹן. אָמַר לָנוּ, לֹא, אִם אֲמַרְתֶּם בְּמֵי גְשָׁמִים, שֶׁאֵין רֻבָּן לָאָדָם, אֶלָּא לָאֲרָצוֹת וְלָאִילָנוֹת, וְרֹב הֶחָלָב, לָאָדָם:
A woman's milk is impure [makes food susceptible to impurity] whether [it comes out] with or against her will, but the milk of an animal is only impure if [it comes out] with its will. Rabbi Akiva says: [The above can be derived] via <i>a fortiori</i> reasoning: If a woman's milk which is only fit for children imparts impurity with or against her will, then is it not logical that an animal's milk which is fit for both children and adults should impart impurity [if it comes out] with or against its will? They [the Sages] said to him: No! If the milk of a woman imparts impurity [if it comes out] against her will, [that is] because blood from her wound is impure [makes food susceptible to impurity], [but] shall milk of an animal [that comes out] against its will be impure when the blood of its wound is pure? He [Rabbi Akiva] said to them: I am more strict with milk than with blood because [if] one milks for healing purposes it is impure, but if one lets blood for healing purposes it is pure. They said to him: Baskets of olives and grapes prove [our point] since the liquids that emerge from them intentionally are impure, but [that which emerges] unintentionally [on its own] is pure. He said to them: No! You refer to baskets of olives and grapes which start as food and end as liquid; will you say the same with milk which starts and ends as liquid? Thus far was the reply. Rabbi Shimon said: From this point on we [the students] argued with him: Rain water proves [the point] since that starts and ends as liquid and only imparts impurity intentionally. He said to us: No! You refer to rain water most of which is not for people but for land and trees, but most of the milk is for people.