Chullin, Chapter 8, Mishnah 5. This chapter has been dealing throughout with the laws of meat cooked in milk and with the borderline cases in which meat and milk come into contact. Our mishnah turns to a substance that sits exactly on that border: the keiva, the rennet. When a nursing calf is slaughtered, the milk it drank is found hardened inside its stomach, and that hardened milk, together with the lining of the stomach itself, contains what is needed to curdle milk and produce cheese. Is that substance milk, or is it flesh? And whose milk is it, the calf's or the mother's? The mishnah presents four rulings, and the Gemara will show that they do not all rest on the same assumption.
The rennet of an animal that is a neveila
The opening case: "Keivat nochri," rennet taken from an animal that a non-Jew slaughtered, and, in the mishnah's next words, "v'shel neveila," rennet from a carcass. Both are off limits: "asura." Slaughter carried out by a non-Jew does not render an animal fit, so such an animal stands in the category of neveila. Granted, the species may well have been kosher, and what sits in the stomach began its existence as ordinary milk. Even so, this ruling treats the congealed milk as belonging to the body that holds it. Once that body is a neveila, whatever counts as part of it falls under the same prohibition, and the rennet may not be used.
Making cheese in the stomach lining
The second case: "Hama'amid be'or shel keiva," one who sets his cheese using the hide of the stomach. That lining carries the curdling agent that turns liquid milk into cheese, so a cheesemaker might well pour his milk into the skin and let it do the work. The difficulty is that the skin is meat. Hence the mishnah's condition: "im yesh ba notein ta'am," if the quantity present suffices to lend a flavor of meat to the milk. In such a case the cheese is prohibited, since meat and milk have now been joined into one food.
A kosher calf that nursed from a tereifa
Next: "K'sheira sheyanka min hatereifa," a kosher calf that took milk from a mother who was a tereifa. Of its rennet the mishnah says "keivatah asura," it is prohibited. Notice that the calf itself drops out of the picture. The congealed milk in its belly originated with the mother, and she was a tereifa, so her milk bore a prohibition on its own account. Even now, lying inside an animal whose own status is unblemished, the substance is traced back to where it came from, and its origin was forbidden.
A tereifa calf that nursed from a kosher animal
Now the mirror image: "Tereifa sheyanka min haksheira," a calf that is itself a tereifa but nursed from a kosher mother. Of it the mishnah rules "keivatah muteret," its rennet is permitted, and it supplies a reason: "mipnei shekanus b'meieha," because the milk is merely collected inside her belly. It went in and was held there, and that is the whole of it. On this reckoning the milk never becomes a component of the tereifa calf; it is as though a separate container of milk happens to be lodged within the animal.
The Gemara's question
The Gemara notices that the first ruling and the third do not seem to work from the same premise. In the first case, the rennet of a neveila was forbidden precisely because we viewed it as part of the animal, and anything that is part of a neveila becomes forbidden along with it. In the third and fourth cases, we ignored the animal in which the rennet is currently found and asked only where the milk came from. If the source was permitted, the rennet is permitted, even while it lies inside the stomach of an animal that is a tereifa. In other words, the later rulings assume that the rennet is not part of the animal holding it at all, which is exactly the opposite of the assumption behind the opening ruling.
The resolution
The Gemara replies that two opinions have been set down side by side in a single mishnah. The first ruling voices the earlier Tannaim, who held that congealed milk in a calf's belly counts as flesh of the calf. The rulings that follow voice a later group of Tannaim who argued against them, maintaining that such milk is no part of the calf at all but resembles milk standing in a container of its own. On their view, rennet would remain permitted even when taken from a calf that is a neveila, provided the milk itself originated from a permitted source.