Bava Metzia, Chapter 1, Mishnah 5. This Mishnah discusses the status of certain types of people who find a lost object, and the question of who acquires it. The Mishnah lists two categories: a list of those whose find transfers to someone else - to a father, a master, or a husband, and a list of those whose find remains in their own hands. We will examine each of the cases separately, as their reasons are completely different from one another.
"Metziyas beno uvito haktanim" - the find of his minor son and daughter:
When the Mishnah uses the term "ktanim" (minors), it does not mean a minor in the usual sense, someone who has not yet reached the age of mitzvos, but rather someone who is in a state called "samuch al shulchan aviv" (dependent on his father's table) - he still relies on his father for his livelihood. Strictly speaking, there is no reason the child should not acquire it for himself. However, the Sages intervened and established: as long as the son is dependent on his father's table, whether he is nine years old or nineteen years old, his find transfers to the father miderabanan.
The reason for the enactment is because of animosity: if the father sees that his son found money and is keeping it for himself, while he himself works hard to support him, he will become bitter and say, "This money is part of the family budget from which I support you." This will create animosity, and the father might even kick him out of his house and stop supporting him. It turns out that the matter does not depend on age itself: one who is not dependent on his father's table - for example, someone who left home at the age of eleven and lives independently - acquires his find for himself, and is not obligated to give it to his father.
Regarding the daughter, the Rishonim argue about the reason for the law:
Because of animosity: Some authorities learn that her law is exactly like the son's law, and the Bartenura suggests this elsewhere - we do not want the father to become angry at his young daughter who lives in his house, and therefore her find transfers to him.
Min Hatorah, based on a derashah: Many Rishonim, as well as the Bartenura here on our Mishnah, maintain that there is a different reason here. The matter is derived from the verse in Parashas Mattos: "Bine'ureha beis aviha" - the daughter is considered to be in her father's domain during her youth, up to the age of twelve and a half, meaning until six months after her bas mitzvah. His authority over her is significant: he may marry her off and accept kiddushin on her behalf, and he annuls her vows when he hears them. Because of this, he also has rights to things that she finds, and her find transfers to him min Hatorah.
The practical difference between the approaches, aside from the actual difference between min Hatorah and miderabanan, is this: if the reason is because of animosity, her law is like the son's law - the matter does not depend on her age at all, but only on the question of whether she is dependent on her father's table. But if the law is min Hatorah, all her finds until the age of twelve and a half belong to the father, whether he supports her or not. The Shulchan Aruch rules that the matter is derived from a derashah, and everything the daughter finds until the age of twelve and a half belongs to her father.
"Metziyas avdo veshifchaso hakena'anim" and "metziyas ishto" - the find of his Canaanite slave and maidservant, and the find of his wife:
Canaanite slaves and maidservants are slaves of non-Jewish origin. They do not have their own property rights, since they are considered the property of the master, and therefore whatever they find transfers to him automatically.
Likewise, a person's wife - her find is his. The reason is similar to the reason given regarding children: because of animosity. Since the husband works hard and supports his wife, there is a concern that he will become bitter toward her if she finds money and buys jewelry for herself with it, and from there things will spiral into arguments. Since we do not want arguments, the Sages enacted that the wife's find transfers to the husband and is included in the family budget. This concludes the first part of the Mishnah.
The second part: "Metziyas beno uvito hagedolim" - the find of his adult son and daughter:
In contrast to the above, a son who is a "gadol" - meaning that he is no longer dependent on his father's table and is financially independent, as well as an adult daughter - halachically, a daughter who has passed the age of twelve and a half, both acquire what they find for themselves.
"Metzias avdo veshifchaso ha'ivrim" - the found item of his Hebrew slave or maidservant:
This refers to a Jewish slave, as opposed to a Canaanite slave. A Hebrew slave is someone who has passed the age of bar mitzvah and got entangled in financial difficulties, and he arrives at slavery in one of two ways:
He is unable to support himself, and out of desperation sells himself into slavery.
He stole and does not have the means to repay the theft along with its penalties, and the Beis Din sells him into slavery in order to pay his debt - a sort of debtors' prison.
In both cases the sale is for a maximum of six years, and if the Jubilee year arrives before that - he goes free. It turns out that even though they are in a state of slavery, their bodies are not acquired by the master but only their time, and their halachic status is like that of any Jew in every regard, including regular property rights. Therefore, a Hebrew slave who found a lost item - acquires it for himself, and it does not pass to his master, since the master is not his owner at all but rather like the employer of a hired worker.
And as for a female, the Hebrew maidservant: This refers to a girl who has not yet reached the age of bas mitzvah, whose father did not have the means to support her and sold her to this family, out of hope that the master who bought her would marry her, or marry her to his son. In this situation she works in the family's house, and the master is not her owner, and therefore her found item does not pass to him.
It is worth noting: The verse teaches that the found item does not pass to the master, but the Gemara says that it passes to her father. This is a great proof that the law is learned from a derivation and depends on the age of twelve and a half, and has no connection to the reason of animosity - for a Hebrew maidservant does not live at all in her father's house and is not supported at his table but rather at her employer's table, and nevertheless the father acquires her found item. And since every Hebrew maidservant has not yet reached the age of mitzvos, her found item always goes to her father - as long as it does not go to her employer.
"Metzias ishto shegeirshah" - the found item of his wife whom he divorced:
Simply understood, the intention is a woman whose husband divorced her and she found a lost item. But this is simple and obvious, since once she is divorced she is under her own authority, and she certainly acquires her found item. Rather, the Mishnah adds: "Af al pi shelo nasan kesubasah - harei elu shelahen" - even though he did not give her kesubah, these belong to them.
The Gemara explains that this refers to a doubtful divorce, with a questionable get: The husband threw the get to her and we do not know if it fell closer to him or closer to her, and we are not sure if she is divorced. In such a situation the obligation is still cast upon him by virtue of the conditions of the kesubah to sustain her and to provide her food until she is divorced properly. And nevertheless, if she found a lost item - she acquires it for herself and does not give it to him, even though he supports her and she is as if supported at his table and dependent on him financially.
And why? Because the entire reason for the rabbinic enactment, that a wife's found item passes to her husband, is in order to prevent strife. This woman is standing to be divorced anyway, the relationship between them has reached its end and there is no more love between them, and thus there is no hope that giving the found item to the husband will improve the situation. It turns out, therefore, that the words "even though he did not give her kesubah" are not to be understood literally, but rather they mean that even though the obligations of the kesubah which have not yet been settled are still incumbent upon him, since he did not divorce her with a complete and absolute divorce - she nevertheless acquires what she found for herself.