Machshirin, Chapter 6, Mishnah 8. Throughout this chapter the Mishnah has been sorting out which fluids qualify as liquids for the laws of tumah, and under what conditions. A central principle in this tractate is ratzon, will: many liquids only render food susceptible to tumah when they were brought forth with the owner's consent. The closing Mishnah of the tractate applies that principle to milk, and preserves for us a full give and take between Rabbi Akiva and the Chachamim.
The opening ruling
The Mishnah's first clause deals with "chaleiv ha'ishah", human milk, and rules that it is "metamei" in either situation alike, "leratzon veshelo leratzon": it makes no difference at all whether a nursing woman drew the milk out on purpose or whether it simply seeped from her by itself. Either way the milk holds the halachic standing of a liquid. It can make food huchshar lekabel tumah, and if the woman herself is tamei, the milk leaving her carries that tumah with it.
Animal milk is not governed by the same rule. Of "chaleiv beheima" the Mishnah says "eino metamei ela leratzon": only milk that a person deliberately drew from the animal counts as a liquid for these halachos. Milk that dripped out on its own, without anyone wanting it, has no such standing.
Rabbi Akiva's kal vachomer
Rabbi Akiva presses an argument from the lighter case to the heavier one, a "kal vachomer". His formulation: "mah im chaleiv ha'ishah", if human milk, which is "eino meyuchad ela lektanim", meant for infants alone, is nonetheless "metamei leratzon" and equally so "shelo leratzon", then "chaleiv beheima", which is "meyuchad lektanim uligdolim", available to the young and the grown, "eino din", is it not the obvious conclusion, that it should be metamei with or without intent? The thrust of the argument: a mother's milk serves only a narrow group of drinkers, babies, and yet the halachah grants it liquid status unconditionally. How much more so a substance that people of every age drink, which should count as a liquid even if nobody meant to draw it out.
The reply of the Chachamim
The Chachamim refuse the inference: "lo". Their reasoning: "im timtzi chaleiv ha'ishah", granted that a woman's milk is tamei even "shelo leratzon", that is only because "dam makasa tamei", blood flowing from an injury of hers is itself tamei, and her milk is treated as belonging to that same category. Shall we therefore rule that "chaleiv beheima" is tamei without intent, when in an animal "dam makasa tahor", blood from its injury is tahor and does not make food susceptible at all? The very rationale that produces the halachah by a person is missing by an animal, so the kal vachomer has nothing to rest upon.
Rabbi Akiva answers
Rabbi Akiva counters: "machmir ani bachaleiv mibadam", I hold milk to a stricter standard than blood. His proof: "shebachaleiv lirefuah tamei", when a woman expresses milk as a remedy, because the milk is causing her pain and has to be released, that milk is still tamei, whereas "hamakah lirefuah tahor", blood taken from a wound for therapeutic purposes stays tahor. Since the halachah is in fact harsher with milk than with blood, blood cannot serve as the yardstick for narrowing the law of milk.
Baskets of olives and grapes
The Chachamim now offer a counter-case: "salei zeisim ve'anavim yochichu", let baskets of olives and grapes decide the question. Liquid seeping out of produce stored that way is tamei if its emergence suited the owner, and tahor if it did not. That, they contend, is the ordinary rule everywhere: ratzon is what decides whether a liquid has halachic weight. A woman's milk is the lone exception, and only on account of the parallel drawn to blood from her injury.
Rabbi Akiva distinguishes
Rabbi Akiva replies that the cases do not match. What holds for "salei zeisim ve'anavim" holds because "techilasan ochel" while "sofan mashkeh": olives and grapes come into being as solid food and end up as juice. Wherever a substance crosses from one category into another, it is the owner's consent that lends the crossing halachic significance. Milk crosses nothing: "techilaso vesofo mashkeh", it is a liquid at the start and a liquid at the finish, so there is no change for an act of will to endorse.
"Ad kan haysa hateshuvah", up to this point was the answer. The recorded exchange between Rabbi Akiva and the Chachamim ends here.
Rabbi Shimon's addition
The Mishnah closes with a remark of Rabbi Shimon: "mikan va'eilach", from this point onward, "hayinu meshivin lefanav", we ourselves would have gone on answering in his presence. He is not reporting anything that was actually said to Rabbi Akiva; he is telling us how he and his colleagues would have pursued the discussion further.
The rejoinder he would have offered: "mei geshamim yochichu", rainwater disproves the distinction. Rain is water at its beginning and water at its end; it passes through no change of category at all, precisely like milk. Even so, rain makes food susceptible only where it was there with the owner's consent. If a substance that stays liquid throughout requires no ratzon, why should rain require it?
And the answer on Rabbi Akiva's side: "lo", rain cannot be compared either. Of rainfall it can be said "ein ruban leadam", the bulk of it is not for people's use, "ela le'aratzos velailanos", but for the soil and the trees. Exactly because water coming down from the sky is not inherently earmarked for a person's benefit, an act of will is needed to establish that this water is indeed serving him. Milk, however, "rubo leadam", exists for human consumption from the start, and requires no such expression of will.
With this exchange the tractate reaches its conclusion. Hadran alach Maseches Machshirin.