Weekly Talmud Learning with Rabbi Mordecai Schwartz, director of Admissions, The Rabbinical School, JTS
Talmud Eruvin 20a
A person should not stand in a public domain and drink from a private one, nor from a private domain to a public domain.
We've seen the idea that carrying from one domain to another is prohibited on Shabbat. This idea is usually (though not always) expressed as "carrying from a public to a private domain" or vice versa. Here we see an additional question rising out of this basic law: What if one stands in one domain and imbibes water from another domain? Is one's body another domain? Is it part of the domain that one's feet are in? Is it part of the domain that one's head is in? Perhaps the Shabbat domains end altogether at one's lips? And what is the spiritual message about the Shabbat and the body that we are supposed to take away from this?
Rashi explains our passage by saying that the prohibition is actually the result of Rabbinic anxiety: our Sages were concerned that if one leaned over into another domain and drank from a vessel in that domain, one might come to walk away with that vessel still in one's hands, thus carrying from one domain to another. This means that our Sages were closing what they saw as a loophole: though they saw the Torah as not extending its Shabbat laws to the basic functions of the body—such as eating, drinking, sleeping, and so on—nonetheless, they knew that performing these functions in a way that involved the possibility of carrying plates, cups, or blankets from one domain to another would violate the sanctity of the Shabbat.
Our bodily functions are exempt from Shabbat violation, but we have a responsibility to perform those functions in a way that upholds the holiness the Shabbat demands.