Eruvin, Chapter 7, Mishnah 9. In Mishnah 7 we learned that if the required measure of food for shitufei mevo'os or for eruvei chatzeiros was lost, in whole or in part, one must add to it and restore it to the proper measure. Rabbi Yosei disagrees on this point, at least with regard to eruvei chatzeiros, and according to some opinions with regard to shitufei mevo'os as well.
The Words of Rabbi Yosei:
Rabbi Yosei says: "Bameh devarim amurim - bischilas eruv" - When do these words apply? At the beginning of the eruv. The measure mentioned in the previous Mishnah - a dried fig per person, or eighteen dried figs for an entire group of people - is required only at the time the eruv is made:
"Bischilas eruv" - at the time the eruv is made, the full measure is required. This is certainly said regarding eruvei chatzeiros, and possibly regarding shitufei mevo'os as well.
"Aval bisheyarei eruv - kol shehu" - but as for the remnants of an eruv, any amount suffices. When the proper measure was there originally and afterward part of it was lost, any amount at all is enough, even the smallest of measures, provided that it did not all disappear. In Rabbi Yosei's view, that is sufficient.
Why Is an Eruvei Chatzeiros Needed in Addition to Shitufei Mevo'os?
The end of the Mishnah deals with a case in which shitufei mevo'os was made, and even so an eruvei chatzeiros is required as well. The question arises on its own: once a shituf has been made for the alleyway, what need is there for an eruvei chatzeiros?
"Velo amru le'arev bachatzeiros ela kedei shelo lishkach hatinokos" - and they said to make an eruv in the courtyards only so that the children should not forget. Chazal did not create the requirement of eruvei chatzeiros in addition to shitufei mevo'os except in order to ensure that the children, the next generation, would not forget the matter of eruvei chatzeiros. They might come to think that there is no need for eruvei chatzeiros, and even in a case where there is no shitufei mevo'os they would mistakenly assume that none is needed. Therefore Chazal instituted that even where there is a shitufei mevo'os, which has in truth already joined all the parties together, a separate eruvei chatzeiros must be made in each and every courtyard, in order to permit carrying in those courtyards.
An Additional Reason in Rabbi Yosei's View:
These words can also be explained as the reason behind Rabbi Yosei's position, that the original measure of food is required only at the beginning of the eruv. Since an eruvei chatzeiros made in addition to shitufei mevo'os is only so that the children should not forget, there is room to be lenient with it, and even if the measure no longer remains one may rely upon it. However, this reason works only according to the opinion that understands Rabbi Yosei's words as referring to eruvei chatzeiros alone, and not to shitufei mevo'os.
In summary: In this Mishnah we learned Rabbi Yosei's position, which distinguishes between the beginning of an eruv, where the full measure is required, and the remnants of an eruv, where any amount suffices. We also examined the reason for the enactment to make an eruv in the courtyards even when a shitufei mevo'os exists - "so that the children should not forget" - and we saw how this reason may also serve as the basis for Rabbi Yosei's leniency regarding eruvei chatzeiros.