Rava said, "if he carried a healthy baby wearing its bag out [of the house], he has violated [Shabbat] on account of the bag . . . " [Problem:] But hasn't he violated [Shabbat] on account of the baby, as well!?! [Solution:] Rava reasons like Rabbi Natan who said, "A living being carries itself."
We have learned that one is not allowed to carry from a private space (such as a home or synagogue) to a public space (such as a street or walkway) on Shabbat. A range of complex Torah and Rabbinic prohibitions and exceptions are wrapped up in this general mitzvah. Here, Rava presents his vision of one such exception. In his view, the Torah does not prohibit carrying children in and out of doors on Shabbat. However, one may not strap a diaper bag to the child and claim to merely be carrying the child, with the bag along for the ride. Carrying the bag in and out of doors is prohibited, says Rava, regardless of the child's role. If one carries the child without the bag, one has not violated the Torah's vision of Shabbat.
What differentiates the child from the bag? The child is alive, the bag is not. "A living being carries itself." Though most parents of small children would disagree, for our Sages, a living creature always lightens the load of its carrier. The Torah's vision of burden does not include other living creatures. As the saying goes, "he ain't heavy, he's my brother."
As Pesah approaches, we should ask ourselves what burdens we carry, which we can set down, and how we carry ourselves in the world.