Tzilta Meraba Meichanita: Rav Yoshia's View
The first mishna in Masechet Sukka presents three criteria pertaining the sekhakh of the Sukka. It cannot be placed higher than 20 amot nor lower than 10 tefachim. In addition it must produce more shade than the sunlight it admits (tzilta merubah meichamta). Ostensibly, these requirements apply to the sekhakh – which as Rashi already notes (2a s.v. veshechamta) entails the essence of the Sukka and lends the Sukka its name (sekhakh = Sukka). The gemara debates whether the final clause - the sunlight/shade quotient might apply to the walls as well. This provocative position – though ultimately rejected - might disclose certain basic functions and identities about the Sukka.
The gemara (7b) cites Rav Yoshia's dissenting opinion that the walls of the Sukka must also produce shade. If a person were to construct a Sukka with glass walls – even if the sekhakh were completely kosher, the Sukka would be invalid. One might have suggested a simple rationale for Rav Yoshia. Conventionally we define the mitzva of Sukka as sitting underneath sekhakh. Admittedly, sekhakh can only be considered a halakhic roof if it produces more shade than the sunlight it admits. However, the mitzva doesn't demand sitting in actual shade. Rather the existence of shade enables the sekhakh to be halakhically valid. If the walls admit sunlight but the sekhakh performs its function the mitzva can still be fulfilled.
Presumably, Rav Yoshia redefines the mitzva of Sukka. A person must actually sit in shade (and not just under shade producing sekhakh) to execute the mitzva. If the shade is eliminated - even if by the sunlight streaming through the walls, the mitzva can no longer be performed. Instinctively we might have determined the machloket between Rebbi Yoshia and the Tanah Kamah to be about the definition of the ma'aseh mitzva- must one sit under sekhakh or actually sit in shade.
When discussing the source for Rebbi Yoshia's position the gemara cites a pasuk and derasha which indicates a very different basis. When describing the parochet which separated the kodesh ha-kodoshim from the rest of the Mikdash the Torah employs the word 'vesakota' (literally you should 'roof'). Even though the parochet actually served the function of a vertical wall it is referred to as a roof. From this syntax Rebbi Yoshia established that a vertical wall is also defined as a roof in halakha and the walls of a Sukka must adhere to the same laws which a roof does - namely tzilta meruba meichamta.
This perspective of the walls of the Sukka as part of the roof or as possessing roof-like qualities is reminiscent of a famous position of the Rambam. The gemara claims that the wood of a Sukka are assure be-hana'ah and mots Rishonim (see for example the Rosh) assume that the prohibition only applies to the sekhakh (in line with Rashi's claim that the sekhakh entails the essential part of the Sukka). The walls – whose only function is to enclose the area and support the sekhakh does not possess any kedusha. By contrast, the Rambam (Hilkhot Sukka 6:15 claims that even the walls possess kedusha and no pleasure may be derived from them. In effect the Rambam defines the entire structure of a Sukka - walls included as part of the cheftza shel mitzva (the object of the mitzva or the essential Sukka). The Rambam broadens this definition to include the enclosing walls.
It should be noted that Rav Yoshia develops the concept of the Rambam to a much further extreme. The Rambam merely extends the halakhic Such to include the walls. Rebbi Yoshia (based upon the pasuk) actually imparts to them a quality, which is normally associated with the roof of a Sukka (the capacity to produce shade). The Rambam did not necessarily view the wall as a quasi-roof. Even as a wall, it participates in the Sukka. Rebbi Yoshia however deems the wall a roof and requires it to produce shade. Tosafot (8b s.v. mechitza) in fact already allude to another potential source for Rebbi Yoshia's halakha. Earlier (6b) the gemara had derived the number of walls necessary to enclose a Sukka from the iteration of the term 'Succot' in the Torah (NOTE: The term 'ba-sukkot' appears only once – Vayikra 23 – but depending upon the spelling might refer to numerous 'Sukkot'). Tosafot claim that by referring to walls as 'Sukkot' (a term generally associated with roof) the gemara itself conveys this function. Ultimately Tosafot reject this source but the similarities between the walls and sekhakh seems to be nascent in the Torah itself.
Having established a basis for Rebbi Yoshia's position we might question this based upon the ensuing gemara. The dominant opinion voiced throughout Masekhet Sukka claims that a Sukka must be a 'dirat arai'- a temporary residence. For example one reason the sekhakh cannot be situated higher than 20 amot is because such a tall structure can no longer be considered temporary. Several tanaim however lodge statements about the Sukka which suggest that they require a Sukka to be a dirat keva – a permanent structure. The gemara (7b) lists these various tanaim and includes Rebbi Yoshia!! Somehow his requirement that the walls not admit light indicates his regarding a Sukka as a dirat keva!! Is this inference merely technical or incidental? Are we to assume that if Rav Yoshia demands walls which produce shade the resultant Sukka is likely to be constructed from solid materials which will probably entail a dirat keva? Could it not be possible to construct a durable Sukka with materials which do admit sunlight?
By drawing this alignment, the gemara might have been suggesting an alternative understanding for Rebbi Yoshia. Rebbi Yoshia demands that a Sukka serve in a similar manner to a house (the quintessential permanent structure) and therefore the walls cannot admit light. Had a Sukka been merely a dirat arai we would only view the walls as necessary to bear the sekhakh. Similar to the walls of a hut or a gazebo the walls of the Sukka would be uni-dimensional – pillars to support a roof. Once however we define a Sukka as dirat keva the walls become multi-dimensional serving to enclose and protect the area and not only facilitate the sekhakh. See especially the Rabenu Yonatan in his comments to Sukka (7b) who draws this analogy between Sukka as dirat keva and Rebbi Yoshia's walls. Based upon this approach we do not define the walls as part of the sekhakh but still can justify Rebbi Yoshia's demand that the walls produce shade. If the walls admit sunlight we can no longer consider this house a dirat keva.
Conceivably, there might be an interesting difference between the two understandings of Rebbi Yoshia. The Ravyah claims that according to Rebbi Yoshia the walls must be constructed from material which is not mekabel tuma (non-foods as well as items which have no designated utility) just as the sekhakh must be comprised of these materials. The Ravyah's extension of Rebbi Yoshia's concept highlights his designation of walls not only as integral to the Sukka but as a semi-roof. If we do not impute the status of 'sekhakh' to the walls (and still require shade-producing potential based upon dirat keva), we should not accept this extra sekhakh demand limiting the materials, which can be used to construct the Sukka.