When the Head and Most of the Torso are in the Sukkah, but the Table is in the House - Eruvin 13b
מסכת עירובין דף י"ג ע"ב
ראשו ורובו בסוכה ושלחנו בתוך הבית
When the Head and Most of the Torso are in the Sukkah, but the Table is in the House
The Gemara here teaches us that, according to Beis Shammai, a person who sits and eats with his head and most of his torso in the Sukkah but with his table in his house, he does not fulfill the mitzvah of Sukkah. Although the prohibition to sit in this manner is rabbinic in origin, the implication of the Gemara is that the person sitting in the Sukkah in this manner does not even fulfill his obligation on a Torah level (see Tosafos to Sukkah 3a. d.h. D’Amar). How can the Rabbis deprive a person of his fufillment of a Torah mitzvah?
Bigdei Sheish suggests that the mitzvah of Sukkah is unique in this respect. This is because the mitzvah of Sukkah is defined in the Torah (Vayikra 23:42) as: בַּסֻּכֹּת תֵּשְׁבוּ שִׁבְעַת יָמִים — In Sukkos you shall sit for seven days. Chazal explain that in this verse: תֵּשְׁבוּ — you shall sit — as תשבו כעין תדורו — you shall sit, as you dwell (see Sukkah 28b et al). Thus, even if a Sukkah is built properly in all its aspects, so long as the sitting therein does not fulfill the definition of dwelling one does not fulfill the mitzvah.
That the Rabbis possess the right to define whether the criterion of dwelling has been fulfilled is derived by Pri Megadim (Pesichah Kolleles to Orach Chaim, beginning of section #3) from the Torah’s statement (Devarim 17:10) that: וְעָשִׂיתָ... וְשָֽׁמַרְתָּ לַֽעֲשׂוֹת כְּכֹל אֲשֶׁר יוֹרוּךָ — And do... and maintain to do according to all they direct you [to do]. Thus, the Rabbis have the authority to determine the definition of dwelling. Hence, in taking into account a possibility that a person whose table is in house may be drawn outside his Sukkah, they determined that this could not be considered a valid form of dwelling, and that the person sitting in such a manner cannot be regarded as fulfilling the mitzvah of sitting in a Sukkah, even on a Torah level.
(ועיין בלשון הר"ן בדף י"ג ע"א בדפי הרי"ף: "...דבעינן סוכה הראויה להיות בה קבע שאין לחוש שימשך ממנה אחר שולחנו...", ודוק היטב.)