The Talmud notes the exemption of carrying a living being, who “carries himself” (chai nosei et atzmo). According to Rashi, this is due to the assistance provided by the carried person, which lightens the load. Tosafot disagree, noting that one may not carry even the lightest item; rather, the exemption comes from the fact that carrying of living beings did not take place in the mishkan.

Tosafot’s point is somewhat difficult; it would appear that Rashi’s intent is not to say that the factor is the lightness of the burden, but the fact that it is not being carried alone. Further, the mishkan standard has never dictated that melakhot only apply to the exact items used in the mishkan. Tosafot, however, did address the relationship of this principle to that of “Shnayim Sh’Asauhu”, by noting that the latter principle requires a situation where both parties can do the task on their own; in this case, the person being carried cannot carry himself, and the carrier can do it even if the person being carried were dead or passive.

The Tosafot HaRosh addresses this point by noting that in this case the situation is different, because without the cooperation of the person being carried, it would be impossible to do the task; therefore, the assistance is significant. The P’nei Yehoshua suggests that the mishkan standard is being applied more rigorously here because of the “inferior” status of the melakhah of carrying (melakhah geruah); therefore, derivative melakhot (toldot) need to match the mishkan exactly.

R. Chaim Brisker (Chiddushim al HaShas) asserts that there are actually two exemptions derived from the same source: Shnayim Sheasauhu, and a “partial melakhah”. Only the first requires that both be capable; the second does not, and “chai nosei et atzmo” comes from that category. (See also Resp. Rabeinu Meshulam Igra, O.C. 8).

R. Moshe Feinstein (Resp. Iggerot Moshe, Y.D. I, 2) explains the position of Tosafot as being that the mishkan only teaches us about a melakhah of hotza’ah in a specific format – where an object will be moved from one domain to the next, and it will be clear from looking at the object in its new location that it was moved by someone. However, with a living being, as that being could have moved on its own power, it is not evident from seeing that being in a new location that someone else did the moving. Using this logic, R. Moshe explains the distinction between an adult and a baby in this realm.

(In regards to the question of one “carrying himself” in a wheelchair, see Resp. Machzeh Eliyahu, 39; Resp. Minchat Yitzchak II, 114; Resp. Har Tzvi, O.C., I, 170. In regards to carrying children, see Resp. Maharsham, VII, 52; Resp. Binyan Shlomo, I, 14 and II, 22; Resp. R. Akiva Eiger, 28; Chazon Ish, 45:3; Resp. Tzitz Eliezer, IX, 34 and XIII, 32; Resp Lev Aryeh, I, 22; and Resp. Teshuvot V’Hanhagot, I, 243.)