Gittin, Chapter 3, Mishnah 3. This Mishnah deals with two subjects: the status of a get that was lost and later found - is there reason to suspect that this is not the same get, but a different one that happens to bear the same names; and the force of the presumption of life, on the strength of which we assume that someone who was alive when the agent set out is still alive.
The get that was lost and found:
"HaMevi get va'avado, metza'o le'altar - kasher, ve'im lav - pasul" - an agent who was bringing a get and lost it: if he found it le'altar, immediately, the get is valid. And if he did not find it immediately, it is invalid, because we suspect that this may not be the same get.
What is the definition of "le'altar"?
The Gemara records a dispute among the Amora'im on this point:
First opinion: As long as no person passed by there - that there is someone who saw that no person passed through that place.
Second opinion: As long as no person stayed there - that there is someone who saw that no person stayed in that place at all.
The Gemara further clarifies that the concern exists only in a place with heavy traffic, where caravans are found and people pass through constantly. But something lost in a place where the public does not travel, even if it is found some time later, gives no cause for concern.
Identifying the get by a distinguishing mark:
Moreover, even in a place where the public travels, if he has a clear distinguishing mark that allows identification, the get is valid, as the Mishnah specifies: "Metza'o bechafisah o bidluskema, im makiro - kasher":
"Bechafisah" - he found it inside a case, and he has an identifying mark on the container in which it was found.
"Bidluskema" - a type of satchel, and here too a mark on the container suffices.
"Im makiro" - that he recognizes the get itself, in addition to identifying the satchel or in place of that.
In each of these situations it is known that this is the very same get, and therefore it is valid.
The presumption of life - three cases:
"HaMevi get vehinicho zaken o choleh - notno lah bechezkat shehu kayam" - an agent who set out with the get at a time when the husband was elderly or ill gives the get to the woman on the presumption that the husband is still alive, since he was alive when the agent set out. But if the husband died before the get was delivered, the get is not valid.
The daughter of an Israelite married to a kohen - a woman whose father is an Israelite and who is married to a kohen eats Terumah by virtue of her marriage. If her husband went overseas, she continues to eat Terumah on the presumption that he is still alive. For if he died and she has no children from him, she may no longer eat Terumah.
One who sends his sin offering from overseas - if a person sent his sin offering from a distant land, it is offered on the presumption that he is still alive, since a sin offering whose owner has died is not brought upon the altar.
Regarding the first case: if the husband did in fact die, she is in any event no longer married. But there is a practical difference with respect to yibum: if he had no children and the get was delivered before his death, she does not fall to yibum. And if the get was not valid and was delivered only after his death, then she does fall to yibum and requires either yibum or chalitzah.
Regarding the third case the Gemara notes a separate problem: a sin offering requires semichah, that the owner place his hands upon it before it is offered, as part of the mitzvah. How, then, can a sin offering sent from far away be offered? The Gemara answers in two ways:
The case is a sin offering sent by a woman, and women are not obligated in semichah.
The case is a bird sin offering, in which there is likewise no semichah.
In summary: In this Mishnah we learned that a get that was lost and found le'altar is valid, and if not, it is invalid out of concern that it may not be the same get; and the definition of "le'altar" is disputed by the Amora'im: that no person passed by there, or that no person stayed there. We further learned that the concern exists only in a place where caravans are found, and that a mark on the container or recognition of the get itself renders it valid even in a place where the public travels. In the second part we examined the force of the presumption of life in three cases: a get handed to an agent while the husband was elderly or ill, the daughter of an Israelite married to a kohen whose husband went overseas, and a sin offering sent from overseas - where the Gemara resolved the question of semichah by explaining that it was a woman's sin offering or a bird sin offering.