Shabbos, Chapter 6, Mishnah 8. The Mishnah deals with an amputee - someone whose leg was cut off - who goes out on Shabbos with his artificial leg. Even in ancient times there were prosthetics designed to be walked on, and they were attached to the stump of the leg. In the opinion of Rabbi Meir he may go out with it, since it is considered his actual shoe - an unusually large shoe. Rabbi Yosi forbids it.
Two approaches in understanding Rabbi Yosi's opinion:
Even though he has a prosthetic device, he also goes out on crutches, and the prosthetic serves only cosmetic purposes. He does not actually lean on it, and therefore it is not considered his shoe the way Rabbi Meir viewed it.
Some understand that according to Rabbi Yosi it really does have the status of his shoe, but there is a concern that he may remove it and carry it four amos.
The law of tumah regarding the prosthetic:
If the prosthetic device has a receptacle, in which they would place soft material so that it would be more comfortable for the stump of the leg - it has the status of a receptacle and it is susceptible to tumah. But if it has no receptacle, it is like a plain piece of wood and it is not susceptible to tumah. It is worth being precise: the receptacle is considered a receptacle because of the soft material placed inside it, and not because of the leg that enters into it.
The reason for this: a receptacle must be able to carry something, in the manner that the Torah compares it to a sack. Just as the sack carries the item placed inside it, so too is required of a receptacle. The prosthetic, however, does not carry the leg; on the contrary, the leg is what carries the prosthetic. Therefore the entry of the leg into it does not make it a receptacle, unlike the material placed inside it.
The supports of an amputee:
The Mishnah continues and asks what the supports of an amputee are. Sometimes a person loses both his legs and walks on his knees, and they would make for him a sort of support, of leather or of wood, for what remains of his legs, so that he could lean on them. These items are tamei with midras tumah if he was a zav, since in this respect they have the status of a shoe: just as a shoe becomes tamei because one walks on it, these too are fit for stepping and walking, and therefore they become tamei with midras.
"Veyotzin bahen b'Shabbos" - he may go out with these supports on Shabbos. "Velo nichnasin bahen ba'azarah" - and even though they are considered shoes, and one does not enter the Azarah of the Beis HaMikdash with shoes, with these entry into the Azarah is indeed permitted, since they are not considered shoes in this respect.
"Kisei v'samcha shelo":
Sometimes they would make for a person whose legs were amputated a sort of chair or small stool attached to him, so that he could push himself forward on it while leaning on his hands. And he also has the supports mentioned above, so that he leans to some extent on his legs as well and not only on the chair beneath him. And their law:
"Tamei midras" - both the chair and the supports are tamei with midras tumah if he was a zav.
"V'ein yotzin bahen b'Shabbos" - one does not go out with them on Shabbos, because they are not firmly attached to his body and can be removed.
"V'ein nichnasin bahen ba'azarah" - and at the same time one does not enter the Beis HaMikdash with them, since from this standpoint we consider them his shoe, and they are not permitted in the Beis HaMikdash.
"Einkatmin":
The Mishnah concludes with the matter of the "einkatmin," which has nothing to do with amputees: a stick that was made in the shape of a donkey, which the jesters would carry on their shoulders and use to perform various kinds of comedy. Its status is tahor, since it is not considered an actual vessel. "V'ein yotzin bahen" - and one does not go out with them on Shabbos, because they are nothing but a burden, a load in every sense, and one does not carry it on Shabbos.
In summary: In this Mishnah we learned the dispute between Rabbi Meir and Rabbi Yosi regarding an amputee who goes out with his prosthetic, and the two approaches in understanding Rabbi Yosi's opinion; the law of tumah regarding the prosthetic, which depends on a receptacle for the soft material, and the reason that the leg itself does not make it a receptacle; the law of the supports - they are tamei with midras, one may go out with them on Shabbos and enter the Azarah with them; the law of the chair and the supports - they are tamei with midras, but one does not go out with them on Shabbos and does not enter the Azarah with them; and the law of the einkatmin - they are tahor, and one does not go out with them on Shabbos because they are a burden.