Chullin, Chapter 7, Mishnah 5. This chapter has been occupied with the prohibition of the gid hanasheh, the sciatic sinew, and here our mishnah turns to a very practical question: what happens when the forbidden sinew finds its way into a pot together with food that is entirely permitted? Does the permitted food remain permitted, and does it matter whether we can still pick the forbidden sinew out of the pot?
When the forbidden sinew can be identified
The mishnah's opening words are "Gid hanasheh shenitbashel im hagidin": the forbidden sinew ended up in a pot together with other sinews of the animal, sinews that carry no prohibition of their own. The halachah is then given in two clauses. The first: "bizman shemakiro, benoten ta'am." Where he recognizes which sinew is the gid hanasheh and is therefore able to lift it out of the pot, the only question left concerning what stays in the pot is whether taste was imparted.
In other words, the question becomes purely a question of flavor. If the permitted sinews in the pot amount to sixty times the volume of the gid hanasheh, we treat the situation as one in which no flavor of the forbidden sinew is discernible in them, and the rest of the pot may be eaten. If the ratio is smaller, let us say there were forty parts of permitted sinew against one part of gid hanasheh, there is no bitul; we assume as a matter of course that the taste of the forbidden sinew passed into the permitted sinews, and they are prohibited.
When it cannot be identified
The second half: "ve'im lav, kulan asurin." If he cannot tell which of the sinews in the pot is the gid hanasheh, then all of them are forbidden, and the quantity of permitted sinews makes no difference whatsoever. Every single piece in that pot might be the actual forbidden sinew, and so we prohibit the entire mixture.
This is a striking departure from what we normally expect. Ordinarily, when a forbidden item is mixed into permitted food, sixty times its volume nullifies it. Here that principle does not help us, and the reason is one we have already encountered in this masechet: the gid hanasheh is a beriah, a complete entity in its own right. When the forbidden item is a whole creation and not a fragment, nullification is not so easily achieved. Even sixty times its volume, and more, will not nullify it. So in this particular case, if he lacks the ability to recognize which sinew is the gid hanasheh while it sits there among all the others, he may not eat from the mixture at all.
The sauce in the pot
The mishnah continues: "veharotev, benoten ta'am." As for the gravy in the pot, that is forbidden only if the gid hanasheh imparted taste to it, and here too the measure is sixty. Notice the distinction the mishnah is drawing. We just established that the sinew itself is not subject to nullification, yet the flavor that seeps out of it is a different matter entirely. Taste is not a beriah. It is not a whole creation, and it therefore behaves like any other forbidden substance. So if the permitted contents of the pot are sixty times the volume of the gid hanasheh, the sauce presents no problem and may be eaten.
A slice of neveilah, a piece of non-kosher fish
The same framework now governs other forbidden foods. "Vechein chatichah shel neveilah": a chunk of neveilah, the flesh of an animal that died without valid shechitah, which found its way in among chunks of kosher meat. And the identical rule covers "chatichah shel dag tamei," a chunk cut from a forbidden species of fish, "shenitbashlu im hachatichot," boiled in a pot alongside chunks of permitted fish.
Why should these follow the rule of the beriah? Because we are speaking of a slice significant enough that a host would set it before his guest. Such a piece carries a status of importance, and just as with the beriah we discussed above, it is not nullified no matter how many permitted slices surround it.
Here the mishnah reaches for the very same wording. "Bizman shemakirin, benoten ta'am": so long as the forbidden chunk is identifiable and can be taken out, the kosher chunks that remain are barred only where its taste passed into them. "Ve'im lav, kulan asurot, veharotev benoten ta'am": but if he cannot distinguish the forbidden chunk from the others, the entire potful is off limits, however large the number of chunks may be, for the suspicion rests equally upon each and every one of them, and a chunk of such consequence never dissolves into a mixture.
The gravy, once again, stands apart. It is forbidden only if the forbidden piece of meat or fish imparted taste to it, and that too is measured at sixty. Where the permitted liquid in the pot exceeds the forbidden piece sixty times over, the sauce is permitted, because when it comes to flavor, bitul does apply.