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.