פירוש על עבודה זרה 69:5

Rava says that he can prove both of these laws from the mishnah in which R. Joshua and R. Ishmael argue as to why cheese made by non-Jews is prohibited. R. Joshua argued that non-Jews curdle their cheese with rennet made from nevelah (an animal not properly slaughtered). To this R. Yishmael responded that the rennet of a burnt offering is permitted. This proves that non-flesh parts of animals from which one may not derive benefit (like the burnt offering) are permitted. Thus dung from an ox sentenced to be stoned is permitted and can be used for betrothal.

R. Ishmael clearly states that it is prohibited to derive benefit from rennet of calves offeredfor idolatry. This proves that the same is true for dung.

The Talmud asks why R. Joshua did not respond that the reason the cheese is only prohibited in eating and not in benefit is that the rennet is no longer visible. This is like the murias (fish brine) into which wine was put. It is prohibited but only in consumption because the wine is not visible.
The answer is an important distinction in halakhah—the rennet is an active agent, it is what makes the cheese curdle. Therefore, even though one cannot see it, it is not nullified.

The verse from Song of Songs seems to be interpreted here in the context of written Torah versus Oral Torah. God says to Israel, your words, i.e. the oral Torah, are more precious than even the words of the written Torah. This was an important message/belief of the rabbis living during this period, for the authority of the oral Torah was an issue of great contention.

According to this interpretation, R. Joshua hinted very subtly to R. Ishmael that he should not ask too many questions about the prohibition of Gentile cheese, for this cheese was a new prohibition of the rabbis. One does not cast doubt on new prohibitions, for this could lead to a people adopting a lackadaisical attitude towards them. One can sense here that there is a larger issue at stake in this mishnah than simply Gentile cheese. The issue is rabbinic authority and how much people respect it.

According to R. Joshua ben Levi, the rabbis prohibited Gentile cheese because Gentiles do not make sure that snakes do not bite their cheese and thereby emit their poison. In other words, Gentile cheese may be poisonous! But we do not tell people that this is the reason for the prohibition because it’s a new prohibition and people may not treat it with all due severity. [One wonders if people would have believed this. If Gentile cheese was frequently enough poisonous, wouldn’t they stop eating it?]

R. Yirmiyah notes that R. Joshua ben Levi’s interpretation does not make sense. According to R. Hanina, snake poison renders it impossible for the cheese to harden or to dry out properly. Thus any cheese that was hardened or dried out properly would be permitted. But we know that all Gentile cheese is prohibited, and thus R. Joshua ben Levi’s interpretation does not make sense.

R. Hanina says that Gentile cheese is prohibited because it may have drops of non-kosher milk in it. Shmuel says it is prohibited because as a curdling agent they use dried skins from the stomach of nevelah animals. This dried skin will have rennet in it.

The Talmud here quotes another sugya from Tractate Hullin. In this sugya Shmuel interprets a mishnah such that it states that any rennet (the stomach enzyme) taken from an animal that was not slaughtered properly is prohibited. So how could he here say that the problem is that the skin of the stomach is the problem.