Halakhah הֲלָכָה

If you find a lost item, there is a mitzvah to return it to its owner. This is called הֲשָׁבַת אֲבֵידָה (hashavat aveidah, returning something lost) and it is first mentioned in our parashah.

כִּ֣י תִפְגַּ֞ע שׁ֧וֹר אֹֽיִבְךָ֛ א֥וֹ חֲמֹר֖וֹ תֹּעֶ֑ה הָשֵׁ֥ב תְּשִׁיבֶ֖נּוּ לֽוֹ׃
If you come upon your enemy's bull or donkey [lost and] wandering, you shall surely return it to him.
Later, in Devarim (22:3), the Torah explains that you should return any lost item that you find, not just a lost animal, and not just if the owner is your enemy.
The Mishnah helps us understand when to perform hashavat aveidah. The main question is: will the owner still be likely to hope to get their item back? If you find something that has identifying signs (like a name on it, or if it has an unusual shape, or if it has unique markings, or if it was placed in a very particular position), then you can assume the owner is hoping it will be returned. You should do whatever you can to try to find the owner.
But if the item you find has no signs, like if it’s a dollar bill on the street (that looks like every other dollar bill in the world, and could have come from anyone), then you can assume the owner probably gives up, so you don't have to return that (Mishnah Bava Metzia 2:1-2).
  • The Shulhan Arukh says that if it looks like an item was placed somewhere on purpose, you should not pick it up and try to return it (Hoshen Mishpat 260:9).
  • If you find a wallet on Shabbat, or any other item that should not be moved on Shabbat, the Shulhan Arukh says to leave it where you found it, even though someone else may take it (Orah Hayyim 266:13).