Machshirin, Chapter 6, Mishnah 7. This chapter is occupied with the question of which liquids count as liquids: which fluids are able to make a food huchshar lekabel tumah, susceptible to becoming tamei, and which fluids are themselves capable of carrying tumah. Our Mishnah presents a list of bodily fluids and other substances that fail both tests.
The Mishnah opens with its heading: "Elu lo metamin velo machshirin", these do not convey tumah and do not effect hechsher. In other words, these substances are simply not regarded as beverages at all. Therefore a food that becomes wet from them is not rendered susceptible to tumah, and even if the fluid itself is tamei it does not pass tumah along to anything else.
The list
"Haze'ah vehalecha serucha vehare'i": perspiration, foul discharge (pus), and excrement. This is how the Rav explains these three terms. Even where these come from a person who is tamei, such as a zav, they will not convey tumah, precisely because the halachah does not treat them as liquid.
"Vehadam hayotzei mehen": blood that issues forth in their company, meaning blood that leaves the body together with a discharge of pus or together with waste matter. Blood by itself certainly has the standing of a liquid, both for transmitting tumah and for producing hechsher, yet when it exits attached to these substances the halachah withholds that standing from it.
"Umashkeh ben shemonah": whatever fluid comes from an infant delivered during the eighth month of pregnancy. Chazal worked on the premise that such a baby has no viability, and therefore his legal standing is not that of one who is alive. It follows that nothing his body secretes has the power to bring about tumah or hechsher.
"Rabbi Yosei omer, chutz midamo": Rabbi Yosei enters one reservation, concerning the infant's blood. In his view blood of an eighth month baby does transmit tumah, because we relate to such a child on the model of an infant who has died, and within that comparison his blood holds exactly the status that blood of any other child would hold.
The waters of Teverya
"Vehashoteh mei Teverya", one who drinks the waters of Teverya, and the Mishnah adds "af al pi sheyotzin nekiyim", even when what leaves him is clear. Those hot springs functioned as a purgative. A person would drink from them and shortly relieve himself, and what came out was nothing but water, unmixed with anything else. Even so, it produces no hechsher. Water it certainly is, but having passed out of the body in this fashion it has forfeited the standing of a drink able to make food susceptible to tumah.
Blood of tamei species, and blood drawn for healing
Next comes "dam shechita babeheima", the blood released when an animal is slaughtered, and equally "bachaya uve'ofot hatemei'im", in the case of a wild beast or of fowl belonging to the forbidden species. Alongside these the Mishnah lists "vedam hakaza lirefuah", blood withdrawn from a vein as a medical treatment; opening a vein was the routine remedy of that era for a wide range of ailments. According to the first Tanna, none of this blood, neither that of tamei creatures nor that taken for a cure, carries the power to convey tumah.
"Rabbi Eliezer metamei be'elu": Rabbi Eliezer declares these capable of tumah. His disagreement is limited to the two final items on the list, which he considers weighty enough to be counted as a liquid, so that in his opinion such blood does transmit tumah and does produce hechsher. The argument thus stands between the Tanna Kamma, who leaves these out, and Rabbi Eliezer, who brings them in.
Milk of a male
"Shimon ben Elazar omer": Shimon ben Elazar teaches "chaleiv hazachar tahor", that when a male produces milk it is tahor. Should a man somehow secrete milk, that secretion carries no status of a liquid. No Tanna in the Mishnah is recorded as taking issue with him, and so the halachah follows his ruling: a fluid of this kind leaves food exactly as it was, not huchshar lekabel tumah.
The thread running through the entire Mishnah is one idea: only what has the standing of a genuine beverage carries the power of a liquid in the laws of tumah. Sweat, waste, the fluids of a child who cannot live, water that has passed through the body, and secretions that serve no one as a drink all fall outside that definition.