Zevachim 72 - 78
“Anything that is ‘fixed’ is seen as 50-50”.
In Hebrew, this principle is called “kol kavua k’mechetze al mechetze dami,” and is one of the important tools we are given in Shas for assigning the status of an item whose status is in doubt. Is the item mutar (permitted) or is it assur (forbidden)? Two other similar principles are called “batel b’rov” (nullified in the majority) and “kol d’parish, m’ruba parish” (all that has separated, has separated from the majority). Our Talmudic Sages use these and other principles to resolve the status of an item whose status is in doubt. The main Chumash sources for these principles are Shemot 23:2 and Devarim 19:11, but the understanding of how and when to apply them is explained in Shas (e.g., Ketuvot 15a and Sanhedrin 79a).
The mishna which begins the eighth chapter of Masechet Zevachim teaches what to do when a forbidden animal becomes mixed with a number of kosher animals that were designated to be offered as sacrifices. For example, the first case is that in which a “chatat that should die” becomes mixed together with kosher offerings. (There are five types of “chatat that should die,” as counted by Rashi, such as a chatat whose owner has already died.) The mishna teaches that the procedure for all of the animals in this mixture is for them all to die — i.e., none may be offered and none may be redeemed after becoming blemished.
We don’t say that the forbidden animal is batel b’rov, says the gemara, because, as a living animal, it has special importance, and is not nullified in the mixture. However, the gemara asks, why don’t we remove one animal from the mixture at a time and say about it: kol d’parish, m’ruba parish? In applying this principle, we would be saying that each one that is removed from the mixture is really from the majority of animals, which are permitted, and thereby permit them to be offered. The gemara answers that if we take one from the herd, the prohibited one is considered kavua and fixed among the permitted ones. Due to this, we look at the result as 50-50, which means that each removed animal is still in doubt as to whether it is permitted or not.
Although the principle of kavua doesn’t deny that the majority of animals are permitted, Chazal interpret a specific verse to teach that the result of a kavua case is 50-50, despite the actual majority that exists. This type of teaching, one that is based on a verse in the Torah, is known as a gezerat hakatuv, a decree of that which is written in the Torah. No further logical explanation is necessary. It is sufficient that
Nevertheless, I have heard an explanation to logically explain the difference between kavua and kol d’parish. Here is an explanation that I heard from Rav Moshe Shapiro, zatzal (to the best of my memory), while learning from him in his kollel nearly forty years ago: When something is “found,” the appropriate question to ask is: Where did this come from? Here the question is: Is this meat from a kosher store or from a non-kosher store? Since the majority of stores in the city are kosher, we apply the statistics of majority and say that it came from one of the majority of kosher stores, and is therefore kosher.
If, however, I bought a piece of meat from one of the stores in the city, and then afterwards forget from which type of store I bought this meat, a completely different question is the appropriate one: “What is this? Kosher or not kosher?” This a binary question with only two answers to consider: kosher or not kosher. Therefore, the number of stores is not a factor, and even if there are a majority of kosher stores we say that the meat is “doubtful-kosher” and forbidden to eat.
A fascinating question is asked with regard to the rule of kavua: The Sanhedrin was composed of judges who were located in a chamber of the courtyard of the Beit Hamikdash called lishkat hagzit. They voted on the cases that they heard, and ruled based on majority. But, since they were in a fixed location when they issued their ruling, why shouldn’t the rule of kavua apply, and every outcome be a 50-50 doubt? An answer to this question is that, although their bodies were fixed in location, their dei’ot — minds and thoughts — are not physical objects that can be considered as existing in a fixed location. (Mordechai)
- Zevachim 73b