Rav Nachman bar Rava said, "Rav said that lighting Shabbat lights is obligatory, while washing one's hands and feet [before Shabbat] is optional. But I say that [washing] is a mitzvah!" How is it a mitzvah? As Rav Yehuda said in the name of Rav: "This was Rabbi Yehuda bar Ilai's practice: on Shabbat eve, [his disciples] would bring him a tub filled with hot water and he would bathe his face, hands and feet. Then he would wrap himself and sit in a fringed (tzitzit) toga and appear like an angel of the Lord of Hosts!"
How do we transcend the physical on Shabbat? Our physical existence is temporary, and our bodies are eventually subject to the most horrendous filth, decrepitude, and rot. Shabbat promises a vision of the world to come, in which we imagine our physical decline halted and even reversed. We light Shabbat lamps to inaugurate this period, and light illumines our spirits as well as our homes. But what about our bodies? How do we prepare our all too imperfect flesh for the holy Sabbath?
In the above passage, we see Rabbi Yehuda bar Ilai embracing the physical in order to transcend it. Rather than ignore his bodily needs and desires, he fullfils them in advance of Shabbat, so that the physical reality of his aging body does not impinge on his Shabbat rest. He bathes in hot water, dresses himself in a fine white toga, and takes on the aspect of a totally spiritual being, an angel of the Lord of Hosts. Though this behavior is not strictly obligatory, Rav Nachman bar Rava gives it the approbation of "mitzvah." Here "mitzvah" does not mean a commandment, but has a looser sense, a behavior that is religiously superlative, the best possible way of keeping the commandments. It is a mitzvah to embrace our physical needs prior to Shabbat, that we may transcend them once Shabbat has arrived.