Machshirin 6:7-8

Machshirin 6:7

The following neither convey impurity nor render food susceptible to impurity: sweat, pus, excrement, blood that comes out with the latter two, and the liquids of a child born after eight months (who is deemed unviable); Rabbi Yosi excludes its blood (i.e., he maintains that the blood of such a child does convey impurity, etc.). If one drinks the water of Tiberias, even if it comes out clean (it doesn’t convey impurity. Nor does) the slaughter blood of non-kosher species of domesticated or wild animals or birds, or blood let for medical reasons. Rabbi Elazar rules these last two things unclean. Rabbi Shimon ben Elazar says that milk from a male is ritually clean.

Machshirin 6:8

Milk from a woman conveys impurity whether it came out intentionally or unintentionally, while cows’ milk only conveys impurity if it was extracted intentionally. Rabbi Akiva said that this is a kal v’chomer (an argument a fortiori): if human milk, which is only consumed by infants, conveys impurity whether it came out intentionally or unintentionally, shouldn’t cows’ milk, which is drunk by both adults and children, convey impurity whether it was extracted intentionally or unintentionally? The Sages answered no: human milk conveys impurity when extracted unintentionally like the blood of a human’s wound conveys impurity. Why should the milk of a cow convey impurity when extracted unintentionally when the blood of its wound doesn’t convey impurity? Rabbi Akiva responded that he treats milk more stringently than he does blood because milk extracted for medical reasons conveys impurity while blood let for medical reasons doesn’t. The Sages sought to bring a proof from baskets of olives and grapes, whose liquid conveys impurity when extracted intentionally but not when exuded on its own. Rabbi Akiva said no: you can’t compare baskets of olives and grapes – which start as a food and end as a liquid – with milk – which starts as a liquid and ends as a liquid. The Sages didn’t argue beyond this point. Rabbi Shimon said that from this point Rabbi Akiva's students debated him, saying that rainwater starts and ends as a liquid and it only conveys impurity when collected intentionally. Rabbi Akiva said no: one can’t compare rainwater – most of which doesn’t go to man but to fields and trees – with milk, most of which is used by man.