1) THE MULTIPLE MITZVOS OF "HASHAVAS AVEIDAH"
OPINIONS: The Beraisa lists several cases in which a person is exempt from the obligation to return a lost object. One of these cases is that of a Kohen who is exempt from returning a lost object that is in a cemetery. This exemption is learned from the word, "v'His'alamta" -- "And hide yourself from them," in the verse, "You shall not see the bull of your brother or his sheep cast off, and hide yourself from them; you must certainly return them to your brother" (Devarim 22:1). Although the simple explanation of the verse is an injunction against hiding oneself from returning a lost object, the Beraisa derives from the unusual wording that sometimes one is supposed to hide himself, such as when he is a Kohen and the lost object is in a cemetery.
The Gemara asks why the Beraisa needs to derive the exemption in this case from the verse. Even without the verse, one would know that a Kohen is exempt from the Mitzvah of Hashavas Aveidah when the lost object is in a cemetery, because the Mitzvah of Hashavas Aveidah is only a Mitzvas Aseh, while the Kohen's obligation to avoid becoming Tamei is both a Mitzvah Aseh and a Lo Ta'aseh. A Mitzvas Aseh does not override an Aseh and a Lo Ta'aseh together.
Why does the Gemara say that the Mitzvas Aseh of Hashavas Aveidah does not override the Kohen's obligation to avoid Tum'ah? There is not even a Mitzvah of Hashavas Aveidah in this case! The Gemara earlier (26b) states that a person transgresses the Mitzvas Aseh of "Hashev Teshivem" only when he picks up the object and does not return it. If he merely ignores the object, he transgresses only the Lo Ta'aseh of "Lo Suchal l'His'alem" (Devarim 22:3). In the case of the lost object in the cemetery, the Kohen never picks up the object. Why is there a Mitzvas Aseh of Hashavas Aveidah at all in this case? (RAMBAN)
(a) The RAMBAN answers that the Beraisa's case is one in which a Kohen already picked up the object (from a place which was not Tamei) in order to return it, and afterwards the wind blew it out of his hands into a cemetery.
(b) Alternatively, the Ramban answers that although the Mitzvas Aseh of Hashavas Aveidah ("Hashev Teshivem") did not take effect (since the Kohen did not pick up the lost object), the Lo Ta'aseh of Hashavas Aveidah ("Lo Suchal l'His'alem") would have overrode the Kohen's obligation to avoid Tum'ah had that obligation been only a Lo Ta'aseh.
(c) The RAN disagrees with the Ramban and asserts that a person who does not pick up a lost object does transgress the Mitzvas Aseh of "Hashev Teshivem." How does the Ran understand the Gemara earlier (26b) which says that when a person waits and does not pick up a lost object until he knows that the owner of the object had Ye'ush, and then he takes the object for himself, he transgresses only "Lo Suchal l'His'alem"? The Ran explains that the Gemara there refers to a person who watches the lost object until the owner has Ye'ush. Such a person does not transgress "Hashev Teshivem" since he is watching the object the entire time, ensuring that nothing happens to it. Even though he watches it for selfish reasons, he is acting in a manner which would ensure the protection of the object and its safe return to its owner. When the owner has Ye'ush, he gives up hope of retrieving his object and effectively lets whoever finds it keep it. On the other hand, when a person sees a lost object and walks away from it, he immediately transgresses "Hashev Teshivem" since he does not guard the lost object at all. Accordingly, the Gemara says that if the Kohen walks away from the lost object he transgresses the Mitzvas Aseh of "Hashev Teshivem."
It is interesting to note that RASHI (DH Hai Aseh) is bothered by a different question. If the Mitzvah of Hashavas Aveidah includes a prohibition of "Lo Suchal l'His'alem" as the Gemara earlier says, then why does the Gemara here not mention that Hashavas Aveidah is both an Aseh and a Lo Ta'aseh, like the Kohen's obligation to avoid Tum'ah? Rashi answers that the Lo Ta'aseh of Hashavas Aveidah does not affect the law that an Aseh does not override an Aseh and a Lo Ta'aseh; that is, it does not make the Aseh more powerful. The Gemara in Yevamos (21a) states that the only type of Mitzvah which can override another Mitzvah is a Mitzvas Aseh, not a Mitzvas Lo Ta'aseh. The Gemara therefore does not bother to mention the Lo Ta'aseh of Hashavas Aveidah. (Y. MONTROSE)
2) "ZAKEN V'EINO LEFI CHEVODO"
OPINIONS: The Gemara concludes that the exemption from Hashavas Aveidah which is derived from the verse of "v'His'alamta" is the case of a "Zaken v'Eino Lefi Chevodo" -- "an old man, and it is not according to his honor."
Although the word "Zaken" commonly refers to an old man, the Gemara in Kidushin (32b; see Insights there) says that it refers to "one who has acquired wisdom," a Torah scholar. RASHI there explains the word "Zaken" is an acronym for "Zeh she'Kanah" Chochmah -- "this one has acquired" wisdom. Does the Gemara here use the word "Zaken" in this context, or in the context of its common meaning ("old man")?
(a) The RITVA writes that the exemption from the Mitzvah of Hashavas Aveidah applies to a Torah scholar. It is only because he represents the honor of the Torah that he is exempt from returning a lost object which is degrading to the honor of the Torah. Similarly, the Ritva in Shevuos (30a) writes that this exemption applies only to a Talmid Chacham and not to a rich man, even though returning such an object may be below the rich man's dignity. This is also the opinion of the RAMBAN.
(b) The RAN argues that "Zaken" refers to both a Torah scholar and any old person, and to anyone else whose honor is such that he would not carry around his own possession like this one (the lost object) in public.
This is also the ruling of the TUR (CM 263), who does not even mention the word "Zaken" when he records the Halachah. He writes simply, "One who finds a lost object and it is disgraceful to him to return it."
The TOSFOS HA'ROSH (32a) adds that women are exempt from the Mitzvah of Hashavas Aveidah because of the precept, "Kol Kevudah Bas Melech Penimah" (Tehilim 45:14).
(c) The RAMBAM (Hilchos Aveidah 11:13) seems to have a third opinion. He writes that "if one is a scholar or an honored Zaken." The PRI YITZCHAK (1:52) infers that the Rambam defines a "Zaken" is not just a Torah scholar, but also an elder. Nevertheless, the fact that the Rambam does not mention an honored rich person implies that he maintains that "Zaken v'Eino Lefi Chevodo" does not exempt an honorable rich person from the obligation of Hashavas Aveidah. (Y. MONTROSE)