Mishnayos Machshirin Perek 6 Mishnah 8

A woman's milk renders unclean whether [its flow is] desired or is not desired, but the milk of cattle renders unclean only if [its flow is] desired. Rabbi Akiva said: there is a kal vehomer argument here: if a woman's milk, which is specifically for infants, can render unclean whether [its flow is] desired or is not desired, all the more should the milk of cattle, which is for infants and adults, should render unclean both when [its flow is] desired and when it is not desired. They said to him: No; a woman's milk renders unclean when [its flow is] not desired, because the blood issuing from her wound is unclean; but how could the milk of cattle render unclean when [its flow is] not desired, seeing that the blood issuing from its wound is clean? He said to them: I am stricter in the case of milk than in the case of blood, for if one milks for healing, [the milk] is unclean, whereas if one lets blood for healing, [the blood] is clean. They said to him: let baskets of olives and grapes prove it; for liquids flowing from them are unclean only when [the flow is] desired, but when [the flow is] not desired they are clean. He said to them: No; if you say [thus] of baskets of olives and grapes which are at first a solid food and at the end become a liquid, could you say [the same] of milk which remains a liquid from beginning to end? Thus far was the argument. Rabbi Shimon said: from here on in we used to argue before him: let rain water prove it, for it remains a liquid from beginning to end, and renders unclean only when [its flow is] desired. But he said to us: No; if you say [thus] of rain water, it is because most of it is intended not for human usage but for the soil and for trees, whereas most milk is intended for human usage.

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: