OPINIONS: The Beraisa teaches that there are four domains with regard to Shabbos: Reshus ha'Rabim (public domain), Reshus ha'Yachid (private domain), "Karmelis," and Makom Petur (exempt area). What does the word "Karmelis" mean, and how does it relate to the domain that it describes?
(a) RASHI (3b, DH Ba'i Abaye) says that it comes from the verse, "Ye'aro v'Charmelo..." (Yeshayah 10:18), which means "fertile field." Such a field is not a place of public travel nor is it a place for the use of a private individual.
(b) TOSFOS (6a, DH Karmelis) explains, based on a Yerushalmi, that "Karmelis" comes from the words "Rach u'Mal," which mean "soft and rollable," referring to grain that is neither very moist nor very dry. Since a Karmelis is neither a Reshus ha'Rabim nor a Reshus ha'Yachid, it is referred to by this hybrid name.
(c) The RAMBAM (in Perush ha'Mishnayos) explains that "Karmelis" means "k'Armelis" -- "like a widow." A widow is not married, but she is also not a Besulah. Likewise, a Karmelis is not a Reshus ha'Rabim, but it is also not a Reshus ha'Yachid.
The ROSH YOSEF explains that perhaps all three explanations are valid, and each one refers to a different type of Karmelis:
1. One type of Karmelis is the Bik'ah, a valley full of grain. It is called a Karmelis because of the word "v'Charmelo" which means "fertile field" (Rashi's explanation).
2. Another type of Karmelis is the Mavoy (alley) and Karpaf she'Lo Hukaf l'Dirah (open field that was fenced-in but not for the sake of residential purposes; Gemara 7a). Mid'Oraisa, such a place has the status of a Reshus ha'Yachid, but the Rabanan decreed that it has the stringencies of both a Reshus ha'Yachid and a Reshus ha'Rabim. It is called "Karmelis" because it is like a woman who was once married (i.e., the field once had a status of a Reshus ha'Yachid), but then lost her status (the field is now considered to have the stringencies of a Reshus ha'Rabim).
3. The Rosh Yosef does not explain the third Karmelis. We may suggest that the third type of Karmeles is, as the Gemara mentions, a pole in Reshus ha'Rabim or a step between Reshus ha'Yachid and Reshus ha'Rabim. Since it is set between, or adjacent to, the two domains, it is represented by the hybrid name, "Rach u'Mal."
We thus find that the three meanings of the word "Karmelis" refer to the three different types of Karmelis!
2) 600,000 PEOPLE: DEFINING A "RESHUS HA'RABIM"
OPINIONS: The Gemara discusses whether an Eruv may be made for a Reshus ha'Rabim to permit carrying there. RASHI (6a, DH Reshus ha'Rabim, and 6b, DH Yerushalayim) defines Reshus ha'Rabim as a major highway between cities, or a major public gathering spot in town. The Gemara later (99a) adds that it must be 16 Amos wide. The Gemara here says that when the Jewish people were in the desert, the desert was considered to be a Reshus ha'Rabim. What was it that made it into a Reshus ha'Rabim at that time?
(a) RASHI explains that when the Jews were in the desert, the desert became a "Makom Hiluch la'Rabim," a place in which many people walk. Rashi understands that a defining property of a Reshus ha'Rabim is that many people actually walk there. There is no fixed number of how many people have to walk there in order for it to be considered a Reshus ha'Rabim. This is also the opinion of the RAMBAN, RAN, RASHBA, and the RAMBAM (see (c) below). (See also (d) in the following Insight.)
(b) TOSFOS (DH Kan) explains that one condition of Reshus ha'Rabim is that 600,000 people are actually found there. That is what made the desert a Reshus ha'Rabim. The opinion that 600,000 people are necessary to make a Reshus ha'Rabim is also the opinion of RASHI in Eruvin (6b and 59a), the BEHAG, and the ROSH (Eruvin 1:8). (Tosfos in Eruvin 6a points out that even though the Jews numbered far more than 600,000 people when we include the number of women, children, and mixed multitude, nevertheless the criteria for Reshus ha'Rabim is established based only on what is written explicitly in the verses.)
(c) The RAMBAM (Hilchos Shabbos 14:1) rules that a desert today is a Reshus ha'Rabim. The Kesef Mishneh cites a responsum written by the Rambam's son, Rav Avraham, who explains his father's opinion. When the Gemara differentiates between a desert in the time that the Jewish people sojourned there and in our time, it means that in our time a desert is a Reshus ha'Rabim, while in the times of the Jewish people's sojourn there it was not a Reshus ha'Rabim (as opposed to the way the other Rishonim understand, that it was a Reshus ha'Rabim when the Jews were there, and it is not a Reshus ha'Rabim in our time). A desert is a Reshus ha'Rabim now because all people are free to walk through it. When the Jews were in the desert, the area in which they were encamped was not free for all to walk through, because it was the Jews' private residential area, and thus it was not a Reshus ha'Rabim. Accordingly, the Rambam's criterion for a place to have the status of Reshus ha'Rabim is that it must be an area that is free for everyone to walk through, even if large numbers of people are not actually found there at any given time.
HALACHAH: The BI'UR HALACHAH (OC 345) writes that a G-d-fearing person should follow the more stringent opinion that does not require a Reshus ha'Rabim to have 600,000 people (a). One who follows this stringent opinion will not rely on a "Tzuras ha'Pesach," or what we commonly call an "Eruv," if it encloses a street that is 16 Amos wide, even though 600,000 people do not use that street. (A "Tzuras ha'Pesach" only transforms a Karmelis, and not a Reshus ha'Rabim, into a Reshus ha'Yachid. According to those who do not require 600,000 people, any public area wider than 16 Amos is a Reshus ha'Rabim).
3) 600,000 PEOPLE -- WHEN?
OPINIONS: According to the opinion that an area needs 600,000 people walking through it in order to be considered a Reshus ha'Rabim (see previous Insight), how often must they be there?
(a) The REMA (OC 345:7) says that the 600,000 people must go through that area every day. (This appears to be based on an inference from the RAN who -- when he argues on the opinion that requires 600,000 people -- says that it is not necessary for "600,000 people to be going through every day.")
The MISHNAH BERURAH (345:24) disputes this ruling and points out that no Rishon actually suggests such a ruling.
(b) TOSFOS and RASHI in Eruvin write that 600,000 people must be "Metzuyim Sham," they must frequent the area. They do not need to be walking through together at one moment. Rather, each person may frequent the area at a different time.
(c) The RAMBAN in Eruvin (59a) adds that even according to those who require 600,000 people, the requirement of 600,000 people applies only to an open square ("Pelatya") where people gather together. However, a highway ("Seratya") is considered a Reshus ha'Rabim even without 600,000 people there, since it leads to a place where 600,000 people gather, and it is used by the public.
(d) The RAN cites the RE'AH who adds that according to those who require 600,000 people, not only is a highway considered a Reshus ha'Rabim because it opens into a place where 600,000 people gather, but any area which opens into a gathering place of 600,000 people is considered a Reshus ha'Rabim. Consequently, the entire desert became Reshus ha'Rabim because of the one place in the desert in which the Jews dwelled. (BI'UR HALACHAH OC 345)