In this week's Parsha, the Torah uses the phrase "אחרי רבים להטת," which Chazal understand as a directive to follow the majority (שמות כ"ג:ב'). The specific situation referred to in the Posuk is one which would come up in the context of a Beis Din. If no unanimous consensus is reached, then Beis Din is to adjudicate the case based on a majority vote. This idea of following the majority is known as the Halacha of רוב"," and has numerous applications even outside of Beis Din.
Two different types of רוב are discussed in Rabbinic literature, namely, a רובא דאיתא קמן, and a רובא דליתא קמן. The former is a רוב in which each position can be easily identified. Several illustrations will be helpful. In the case of Beis Din handling a monetary dispute, three judges are required. We poll each of the judges. If two feel that the defendant is guilty and one feels that he is innocent, the decision of Beis Din is in favor of a guilty verdict, because that is the opinion of the רוב. Similarly, should there be a meat market with many different proprietors, and a piece of meat is found without an identifying mark, the question arises as to whether or not this meat can or cannot be considered Kosher. The Halacha states that we resolve this ספק, this doubt, based on רוב. If a majority of the stores in that meat market sell Kosher meat, a Jew may eat the meat based on the assumption that כל מה דפריש מרובא פריש, meaning, in this case, that this piece of meat most likely came from a Kosher store because there are more Kosher stores than non-Kosher stores. Hence, if ten stores, for example, are selling meat and six sell Kosher meat, a Jew is permitted to consume the meat found in that place, because we examine the רוב. A third application of the Halacha of רוב is in a case when it is necessary to determine whether תפילה בציבור can take place. If ten people are present but several people have already davened, the determining factor is that if the רוב, meaning six individuals, have not yet davened, one can conduct a full fledged Minyan.
In these types of cases, the רוב is "מכריע," it decides, the ספק, meaning that we acknowledge the presence of a doubt, and in the absence of definitive information, we rely on the רוב to resolve the dilemma. If, however, it is possible to verify and clarify the matter without relying on the principle of רוב, there certainly exists an obligation to do so. Consequently, in the case of the meat, for example, one should initially survey each store to ascertain whether anybody is aware of having lost a piece of meat. Only if such efforts prove fruitless, or if the proprietors are not available may we rely on the Halacha of רוב to resolve the doubt.
The second type of רוב is a רובא דליתא קמן. In this situation, no accurate count takes place, but rather the decision is based on an observation of a pattern of behavior or a specific situation. Several illustrations will again be helpful. If a Shochet slaughters an animal and then disappears, we have no way to absolutely determine that this individual was indeed a qualified Shochet. We can not know whether or not he was aware of all the laws of Shechitah which, if not carefully considered, may render the animal a נבילה and therefore unfit for consumption. The Halacha states "רוב מצויין אצל שחיטה מומחין הם," meaning that the majority of those people who choose to slaughter are probably qualified to do so. This is similar to the idea that if one were at a ball game and someone collapsed, and an announcement is made seeking and summoning a doctor, there is a presumption that any individual who comes forward is either a doctor or a member of an EMT staff. We do not suspect that a lawyer has come forward and is playing doctor. Similarly, the Halacha feels that in all probability, a Shochet is qualified and one can therefore eat this meat whose status was in doubt.
A second example of this type of רוב is seen from a case where יבום must be performed. According to Torah law, a father has the right to marry off his daughter on his own until she reaches the age of twelve, and the Kiddushin, the marriage, is valid MideOraisa. If a father indeed exercises this option and the husband subsequently dies without having any children, the question of יבום now arises. Normally, this girl would be זקוקה ליבום, that is, she is bound to marry her brother-in-law. The purpose of יבום, though, is "להקים שם לאחיו," meaning, to provide a child who will now carry on the name of the deceased brother and receive his inheritance. In this case, however, there exists a possibility that this young girl may become an איילונית, a woman that never matures physically to the point that she can bear children. Since the purpose of יבום is procreation and this woman is incapable of reproducing, the physical union between herself and her brother-in-law would be prohibited because the Torah prohibits a man to sleep with his brother's wife, except in a case where יבום can be performed. Nevertheless, despite this possibility, we allow the girl in our case to undergo יבום based on the fact that רוב נשים מתעברות, meaning that the majority of women do physically mature and develop the ability to conceive a child. This type of רוב is not just a מכריע, resolving the doubt, but is rather in effect a מברר, meaning that we act as if the doubt does not really exist any longer.
This Halacha of רוב prevails in many other areas as well. When a Kosher animal has been slaughtered, one should theoretically check the entire animal to see if any of the סימני טריפות, the physical signs which would render the animal's meat unfit for Kosher consumption, are present. However, sinceרוב בהמות , most animals, do not have any of these problems, serious examination is reserved for only the animal's lungs, where the statistical probability of having a problem that makes the animal a טריפה represents a significant minority, a מיעוט המצוי, and therefore careful investigation is necessary. Other problems that can render the animal a טריפה are far less common and therefore the same kind of thorough examination is generally not done. This type of רוב also enables us to identify the father of a given child. Biologically, we can ascertain only who the mother of a particular child is. (Genetic testing is not taken into consideration in this study.) However, the husband of the mother is generally identified as the child's father because רוב בעילות אחר הבעל, meaning that the רוב indicates that it is the woman's husband who has fathered the child.
There is one opinion among the Tannaim who believes that in general we can not rely on the Halacha of רוב. In the aforementioned case of the קטנה, the girl under the age of twelve who was married but whose husband died, this Tanna holds that the girl should not undergo יבום (as a קטנה) because of the possibility that she will develop into an איילונית. This Tanna is concerned with the מיעוט, the minority possibility. However, even this Tanna admits that if the minority is insignificant and is thus labeled as a מיעוטא דמיעוטא, we would then not be concerned with it and we would determine the Halacha based on the רוב.
This רוב also serves as the Halachic basis of the concept of a חזקת כשרות, Halachic trustworthiness. Generally speaking, any person who is committed to the system of Mitzvos is entitled to Halachic trustworthiness in all areas of Halacha. For this reason, if someone is Shomer Shabbos, one has the Halachic right to trust his Kashrus. Only if one is a מומר לחלל שבת, a consistent flagrant violator of Shabbos, or a מומר לעבודה זרה, a known idolater, does he forfeit Halachic trustworthiness in other areas of Halacha. This חזקת כשרות is predicated on the fact that if the majority of people in a group are committed to a particular system, then any individual in question who is part of that group is probably committed to that system. It is precisely for this reason that some Poskim claim that today, we can not presume that a חזקת כשרות applies to any specific individuals, for after all, the majority of Jews at present fall into the category of "yet to be observant" Jews.