We begin the final chapter of Maseches Eruvin. This chapter does not deal so much with matters of eruv, but rather with matters of Shabbos. Since this is where the two tractates connected to Shabbos - Shabbos and Eruvin - come to a close, a variety of different topics were gathered here as a conclusion to both of them together.
One Who Finds Tefillin in a Public Domain on Shabbos:
The Mishnah discusses someone who found tefillin lying in a public domain on Shabbos. They may not be left unguarded, since they might fall into the hands of people who would treat them disgracefully, or they might be damaged by animals and the like, and therefore one must act to rescue them. The Mishnah teaches that the way to rescue them is to bring them in by wearing them: he places the shel yad on his arm and the shel rosh on his head, brings them into a private domain, goes back and takes the next pair, and so on in this manner, pair after pair.
Rabban Gamliel disagrees and holds that one may bring in two pairs at once: he places two shel yad on his arm and two shel rosh on his head, and brings them in by wearing them in this way.
The Root of the Dispute - Is Shabbos a Time for Tefillin:
The Gemara explains that the dispute, whether he brings them in one pair at a time or two pairs as Rabban Gamliel holds, depends on the question of whether Shabbos is a time for tefillin, that is, whether there is a mitzvah to put on tefillin on Shabbos:
According to the first Tanna: Shabbos is a time for tefillin and there is a mitzvah from the Torah to wear them, except that the Sages decreed not to put them on lest one come to carry them. Under these circumstances, in order to rescue the tefillin, the Sages were lenient and permitted bringing them in by wearing them. Even so, one may put on only one pair at a time, for since this is a time when one is obligated in tefillin from the Torah, putting on two pairs is an addition to the mitzvos and he transgresses "bal tosif." The law of "bal tosif" applies specifically at a time fit for the fulfillment of the mitzvah, and therefore here, at a time for tefillin, there is a concern of a prohibition.
According to Rabban Gamliel: Shabbos is not a time for tefillin even from the Torah, and there is no concern of "bal tosif" here; nevertheless it is permitted to bring them in, because placing them on the arm and on the head is considered like wearing a garment. And the reason it is permitted to put on two pairs at once is that the Sages said there is room on the head and on the arm to place two pairs of tefillin, and therefore this too is a manner of wearing, and it is permitted to bring them in this way.
"Bameh devarim amurim" - With Old Ones and With New Ones:
"Bishanos" - with old ones: the permission to bring them in was stated regarding old tefillin, that is, tefillin in which it is evident that they have been used and that they are made in the form of tefillin: they have straps, and the straps and knots are tied in the way that tefillin are tied.
"Bachadashos" - with new ones: but if they are new, meaning that they have no straps and they merely resemble tefillin in appearance, he is exempt from bringing them in and may leave them in their place. The reason: in their times people would make amulets, even in the form of tefillin, and one may presume that this is nothing more than an amulet, which has no sanctity at all - and for that reason the Sages were not lenient to permit bringing it in on Shabbos.
"Machshich aleihen u'meviyan" - With a Large Quantity of Tefillin:
And what is the law when they are valid tefillin, but many pairs were found, whether wrapped and tied each pair separately or single tefillin, and they are so numerous that not enough time remains on Shabbos to bring them all in by wearing them? Regarding this the Mishnah says "machshich aleihen u'meviyan" - he waits there until dark and brings them: he will remain there, guard them so that no harm comes to them, and after Shabbos ends he will bring them. Since it is impossible to bring them all in on Shabbos, the Sages were not lenient at all, and said that it is better that he remain with them to guard them and bring them after Shabbos.
"Mechasan v'holech lo" - At a Time of Danger:
The Mishnah adds that if it is a time of danger, such as when the government decreed, Heaven forbid, that tefillin may not be worn, and there is a risk in putting them on and bringing them in this way - "mechasan v'holech lo" - he covers them and goes on his way. He covers them with something, such as straw, so that they will not remain in a state of disgrace; even though they remain in their place, they are covered and tucked away to the side, and he goes on his way and returns after Shabbos to carry them to his home.
The Gemara adds a distinction within a time of danger:
Danger from non-Jews: the law of covering was stated when the danger stems from a government decree against wearing tefillin. Since it is even forbidden to be seen with tefillin, there is no other option but to cover them, return after Shabbos, and take them all at once.
Danger from bandits: but when the danger comes from robbers, he cannot remain in that place until after Shabbos and wait there alone, and he must leave his place. In this case he takes a sack and gathers all the tefillin into it. And how will he carry them, given that they are too many to place on his body? He carries them by moving less than four amos, resting, then walking less than four amos again, and so on. In this way he does not transgress the Torah prohibition of carrying in a public domain; and even though there is a rabbinic prohibition involved, the Sages were lenient and permitted this for the sake of rescuing the tefillin.
In summary: in this Mishnah we learned the ways of rescuing tefillin found in a public domain on Shabbos: bringing them in by wearing them, one pair according to the first Tanna or two pairs according to Rabban Gamliel, with their dispute depending on the question of whether Shabbos is a time for tefillin and on the concern of "bal tosif." We also learned the distinction between old and new ones due to the concern that they may be an amulet, the law of "machshich aleihen u'meviyan" with a large quantity of tefillin, and the law of "mechasan v'holech lo" at a time of danger - with danger from non-Jews, by covering them and waiting until after Shabbos, and with danger from bandits, by carrying them less than four amos at a time.