Machshirin, Chapter 2, Mishnah 9. The later mishnayos of this chapter deal with a practical question: when something is found in a city whose population is mixed, part Yisroel and part nochri, how do we determine its halachic status? The answer the Mishnah gives repeatedly is that we follow the majority, and here that principle is applied to a piece of meat.
"Motza bah basar, holchin achar rov hatabachim." One who finds meat in such a city follows the majority of the tabachim, the butchers, meaning in this context those who slaughter and sell meat. If most of the shochtim in that city are Jewish, the meat is mutar.
What exactly are we worried about?
The concern here is not that the meat might come from a non-kosher species. That is something a person can establish by examining the piece itself. The open question is whether the animal received a proper shechitah, and that depends entirely on who did the slaughtering. Since we have no way of identifying him, we lean on the makeup of the trade in that city: if the majority of those who slaughter meat there produce kosher meat, we may assume that this piece too was properly slaughtered.
The problem of meat left unwatched
The Rav raises an objection from another halachah. Chazal enacted a prohibition called "basar shenisalem min ha'ayin": once a piece of meat has slipped out of sight, so that we can no longer trace what became of it, it is forbidden, out of suspicion that it was switched for meat that is not kosher. This gezeirah is taught by the Amora Rav. At first glance our Mishnah seems to contradict him, since meat discovered lying in the street is precisely a piece that nobody was guarding, and nevertheless the Mishnah permits it based on the majority.
The Rav, the Rishon, explains how Rav, the Amora, would have understood our Mishnah so that it fits with his own decree. Rav would read the case as follows: a person saw from a distance someone slaughtering an animal, and he could not make out whether the slaughterer was a Yisroel or a nochri; he then watched him set the meat down, and the meat never once left his field of vision. In such a case there is no worry that the piece was swapped for something forbidden, since it was under observation the entire time. The only doubt is whether the shechitah was valid, and for that we rely on the majority of slaughterers in the city.
Al pi pshat, however, the Mishnah is read more simply: it is speaking of ordinary found meat, and the concern of basar shenisalem min ha'ayin is not being raised at all.
Cooked meat
"Im hayah mevushal": if the meat that turned up was already cooked, then the majority we consult is that of the "ochlei basar mevushal," those who eat cooked meat. The Mishnah is not distinguishing between people who cook their meat and people who consume it raw, which is hardly a normal habit. It means rather the bulk of the town's residents who buy meat and eat it. Should most of them be Jews, we treat this portion as permitted; should most of them be nochrim, we must assume the reverse, and the piece is assur.
In both halves of the Mishnah the same tool is at work. Where the identity of the person behind the food cannot be established, the Torah allows us to build on the character of the majority: in the case of raw meat, the majority of those who do the slaughtering, and in the case of cooked meat, the majority of those who eat it.