Keritot 13B

וחלב אשה ⁠– מטמאין טומאת משקין ברביעית. רוקו, זובו, ומימי רגליו ⁠– מטמאין טומאה חמורה, בכל שהוא.

and the milk of a woman who is a zava transmit impurity of liquids where there is 1/4th-log. The saliva, gonorrhea-like discharge of a zav, and urine transmit severe impurity in any amount.

ואי אמרת מקום חלב מעיין [הוא], חלב נמי נטמא טומאה חמורה בכל שהוא, כזובו ורוקו! אלא שמע מינה: מקום חלב אשה לאו מעיין הוא.

And if you say in accordance with the opinion of Rava that the location from which the milk emerges is considered a spring, then milk also should transmit severe impurity in any amount, like the gonorrhea-like discharge of a zav and his saliva. Rather, conclude from it that the location from which the milk of a woman emerges is not considered to be a spring, and the milk must be rendered susceptible to impurity in order to become impure or transmit impurity.

אי הכי, קשיא הא מתניתא ואמר רבא: ״מטמא בין לרצון ובין שלא לרצון״!

The Talmud objects: If so, this Mishnah in Makhshirin cited earlier (13a), which Rava said supports his opinion, is difficult, as it states that a woman’s milk renders food susceptible to impurity whether it emerged to the satisfaction of the infant or not to his satisfaction. The Mishnah is difficult, as food is generally rendered susceptible to impurity only when the liquid comes into contact with it to the owner’s satisfaction.

מי סברת ״שלא לרצון״ דאמר ⁠– דלא ניחא ליה? לא, מאי ״שלא לרצון״ דאמר, דדעתיה דתינוק קריבא לגבי חלב, אבל אמר לא ניחא ליה ⁠– טהור.

The Talmud explains: Do you maintain that the term: Not to their satisfaction, that the Mishnah states, means that the emergence of the milk is not amenable to him? No; rather, what is the meaning of the expression: Not to his satisfaction, that the Mishnah states? It means that the child did not indicate whether he desires the milk or not, but as a child’s mind is close to milk, i.e., he generally enjoys the milk, an explicit indication of interest or satisfaction is unnecessary in order for the milk to be susceptible to ritual impurity or to render other food items impure. But if he says, i.e., indicates, explictly that the milk is not amenable to him, then the milk is not susceptible to ritual impurity, and it remains pure.