Gittin, Chapter 5, Mishnah 5. This mishnah appears in Maseches Eduyos as well, since it contains testimony on several different matters; but the last two matters in it deal with things that were instituted for the sake of tikkun ha'olam (the good order of the world), and that is why they belong here in our chapter.
"Eilu ha'eiduyos she'he'id Rabbi Yochanan ben Gudgeda" - these are the testimonies that Rabbi Yochanan ben Gudgeda testified to:
"Al ha'chereshes she'hisi'ah aviha she'hi yotz'ah b'get" - a deaf-mute woman whose father married her off: her kiddushin are valid by Torah law, and nevertheless she goes out with a get. The reason: by Torah law a divorce takes effect for a woman even against her will, and therefore it does not require her understanding or her mental capacity, and a deaf-mute woman is generally regarded as one who lacks understanding. Even though her kiddushin are by Torah law, her divorce is valid, and the get is placed in her own hand without her father's involvement.
"Al ketanah bas Yisrael she'niseis l'kohen she'ocheles bi'Terumah" - this speaks even of a minor girl whose father is no longer alive, and whose mother or brothers married her off, so that her marriage and her kiddushin are only rabbinic. Even in these circumstances she eats Terumah, that is, rabbinic Terumah, and we do not decree against it or worry that she may come to eat Terumah of Torah law.
"V'im meisah ba'alah yorshah" - regarding that same minor girl and those same rabbinic kiddushin: if she dies, her husband inherits her. Even though by Torah law others are her heirs, the Sages, by the power of "hefker beis din hefker" (the court's authority to declare property ownerless), made these rabbinic kiddushin into full kiddushin with regard to inheritance.
"V'al ha'marish ha'gazul she'bena'o b'birah she'yitol es damav":
A large beam that was stolen and built into a large structure, into a mansion. By strict law the robber is obligated to return the stolen object itself, but since he has already built it into the structure, and he now wishes to repent, it would be exceedingly difficult for him to dismantle the entire building in order to return the beam. Therefore the Sages instituted, for the sake of takanas ha'shavim (encouraging penitents), that the victim takes the value of the beam in its place, even though by strict law the beam itself should have been returned to him.
"V'al chatas ha'gezulah she'lo nod'ah la'rabim she'hi mechaperes":
A chatas (sin offering) animal that was stolen, where the matter is not publicly known, so that the kohanim are unaware that it is stolen - it effects atonement and is valid as an offering. We do not view it as chullin (unconsecrated animals) in the Temple courtyard, as though an unconsecrated animal had been brought into the Beis Hamikdash, whose offering is void and whose meat is forbidden to eat.
This enactment was established for the sake of the good order of the Altar, even though by strict law it should not have been effective. For if the kohanim were required to investigate the source of the offerings, they would worry that they might have come from unreliable sources and that the animal might be stolen, and as a result they would refrain from offering the sacrifices and from eating them. In order to ensure that there would be kohanim ready to perform the Temple service and to offer sacrifices upon the Altar, the Sages instituted - by the power of their authority in monetary matters and by the power of "hefker beis din hefker" - that in such circumstances the offering belongs to the robber.
In summary: in this mishnah we learned the testimonies of Rabbi Yochanan ben Gudgeda: a deaf-mute woman whose father married her off goes out with a get, since divorce does not depend on her understanding; a minor Israelite girl married to a kohen by rabbinic law eats rabbinic Terumah, and her husband inherits her by the power of "hefker beis din hefker"; a stolen beam that was built into a mansion - the victim takes its value, for the sake of encouraging penitents; and a stolen chatas that was not publicly known effects atonement, for the sake of the good order of the Altar.