Up to this point, there have been several Mishnas dealing with saving items on Shabbat. This is the first Mishna that actually mentions the concept of eruv. Why was this not discussed earlier? At first a chatzer was mentioned, then a mavuy, and now eruv.
Daniel Ettedgui, Boca Raton, USA
Shalom Rav,
You write, "At first a Chatzer was mentioned, then a Mavoy, and now Eruv." This is not quite correct. The first and second Mishnah (which are one Mishnah in the Mishnayos), both of which speak about saving Kisvei Kodesh, are talking about saving them in a Mavoy (as the Tana states in the second Mishnah).
The first Tosfos Yom Tov in the Perek explains that since the Tana (in the second Mishnah) makes no mention of Eruv whatsoever, he must be speaking about where there is no Eruv (and no Shituf Mavu'os).
The third and fourth Mishnah on the other hand -- which speak about saving food -- permit saving food specifically in a Chatzer that has an Eruv, as the Tana explains at the end.
B'Virchas Kol Tuv,
Eliezer Chrysler