I
"And he took the knife to slaughter his son” (Breishit 22:10)
Rebbe (Chulin 16a) derives from this verse a halacha with regard to the knife that may be used for the shechita of an animal or bird: It cannot be attached - ostensibly meaning - to the ground. Accordingly, a sharp plant or branch - even if totally smooth - cannot be used for shechita if it is still attached to the ground. Tosfot (ibid.) explains that the deduction is based upon the Torah having said that he "took" the knife as opposed to saying that he "prepared" the knife. [See Rashi ibid.) This implies that a knife used for shechita must be an item which can be taken, i.e. transported; it is not attached and stationary.
The Mishna (Chulin 15b) teaches us that one may not slaughter an animal or bird with his fingernail. Although, there were sacrificial offerings of birds that were slaughtered by the kohein with his fingernail, (a procedure known as melika), that pertains solely to a kohein officiating by a sacrifice. However, when performing a shechita upon regular meat - even a bird - one may not use his fingernail. Why not? The Gemara (Chulin 18a) explains that this is due to the very same rule of Rebbe which prohibits slaughtering with an attached item and the nail is attached to the person’s body.
How, though, can this be a valid explanation? Didn’t we mention that the nature of Rebbe’s rule is to exclude something attached to the ground….a person is not ?????
II
R’ Akiva Eiger (Yoreh Deah Chapter 6, Responsa 51) gives an exlanation based on a well known rule of the Gemara (See Kiddushin 22b) which states that a slave is compared to landed property. Accordingly, slaves are governed by some of the same laws as land. For example, just as certain oaths are not administered regarding disputes about land, they are similarly not administered regarding disputes about slaves. This equation relates to many other examples throughout Shas. Rishonim (Rashi and Tosfot Kiddushin 7a) discuss whether this comparison applies specifically to slaves or to all human beings. The Shach (Choshen Mishpat 95) rules that all humans have this principle.
Accordingly, the law which invalidates an item attached to the ground from being used for slaughtering can very well apply to a fingernail attached to one’s finger, just as it does to a branch of the rooted tree. However, this answer only follows the ruling of the Shach. According to the other opinion we have only invalidated the fingernail of a slave. We still have no explanation of how to invalidate the fingernail of a regular person.
R’ Akiva Eiger reasons that a rabbinical decree must be responsible for this prohibition. In order to avoid the use of the nail of a slave, Chazal invalidated anyone’s fingernail. In fact, though, a regular person’s fingernail may be used for slaughtering by Torah law.
III
Shulchan Aruch (Yoreh Deah 6) rules based that a nail of an animal may not be used for shechita. The ruling is based on the very same principle that we have been discussing, namely, that it is an attached item. In this case, however, it is attached to an animal. This extension from man to animal troubles R’ Akiva Eiger: how can Rebbe’s principle pertain to that which is attached to an animal? By no means are animals equated with land! Rather, we must assume that this is also a rabbinical decree lest one use a human (or slave - see above) fingernail which is truly considered to be attached to the ground. However, asks R’ Akiva Eiger, where is the source for such a decree? The Gemara never mentions such a decree regarding animals. How, then could the Shulchan Aruch create a prohibition on its own?
IV
Ohr Sameach (Hilchot Gerushin, Perek 1) differs with the entire approach of R’ Akiva Eiger (in section II). He claims that the principle which compares people to land is totally not applicable in the context of shechita. This principle is used in the Gemara only in monetary issues, such as oaths in disputes, methods of acquisition and the like. There is no such equation for issues which fall under the category of issur such as shechita. Accordingly, even the invalidation for slaughtering of a human’s or slave’s fingernail cannot be based upon that principle.
Rather, we must assume that the prohibition regarding slaughtering with an item that is "attached" is not limited to that which is attached to the ground. Instead, we must understand that the prohibition invalidates any item that is simply "attached". As such, a fingernail of a person or an animal is also deemed invalid.