After mentioning all the chidushim and limudim derived form the various items that are stated explicitly in the Torah by eish, the gemara asks that it would seem to be enough just to write Sadeh alone. The question is obvious, how would i know then to exclude Tamun unlike Rabbi Yehuda, or that even for a hefsed muot is one chayiv?
Aschi, Toronto Ontario
Baruch she'Kivanta! Your question is asked by the Pnei Yehoshua!
(1) The Pnei Yehoshua writes that it would be forced to suggest that the question of the Gemara only applies according to Rebbi Yehudah and therefore no other word is needed to teach that Tamun is exempt.
(2) Therefore the Pnei Yehoshua suggests an ingenious solution. He writes that even if the Torah would have only written "Ki Setzei Esh u'Mav'ir Shalem Yeshalem" - "If fire breaks out and devours, you shall surely pay" - this would have been sufficient to include every possible damage (including Hefsed Mu'at, which answers your second question). There would have been no reason to suggest that this did not include damage caused to thorns, or the scorching of the furrow in the field, because this is all included in "fire breaks out and devours".
So when the Gemara asks that the Torah should only write "Sadeh", all this means is that the word Sadeh would be sufficient to teach that Tamun is excluded, but all other damages would automatically be included.
PY adduces a beautiful proof that there would be good reason to suggest that "Sadeh" excludes anything hidden. This is from Chulin 72a where the Gemara explains that the verse (Bamidbar 19:16)
"And anything which should touch [the impure object] on the face of the field - Sadeh" will become impure, comes to exclude a fetus in the mother's womb, that does not become impure. Rashi there DH Al Pnei writes that "on the face of the field" suggests something in the open, whilst a fetus is hidden. One learns from the Gemara Chulin 72a that "Sadeh" means "revealed", so this would be a good source to teach that Tamun is exempt from the damaged caused by fire, and this is all the Gemara means when it asks that the Torah should only write Sadeh.
On this the Gemara answers that in fact one could not have derived the Ptur of Tamun from "Sadeh" because on the contrary I would have said that Sadeh only includes what is in the Sadeh, i.e. produce as Rashi explains, whilst one might have thought that one is exempt for damage done to the land itself. This is why the word "Kamah" is necessary - to teach that the word Sadeh in fact means land. If so, we do not now have a word to prove that Tamun is exempt, which is why the Torah writes "Gadish" which leaves "Kamah" spare to teach that Tamun is exempt.
Shavu'a Tov
Dovid Bloom
After mentioning all the chidushim and limudim derived form the various items that are stated explicitly in the Torah by eish, the gemara asks that it would seem to be enough just to write Sadeh alone. The question is obvious, how would i know then to exclude Tamun unlike Rabbi Yehuda, or that even for a hefsed muot is one chayiv?
Aschi, Toronto Ontario
Follow-up reply:
Here is an alternative answer to your question. One can answer that the Gemara is asking specifically according to Rabbi Yehuda. Even though the Pnei Yehoshua mentioned this possibly but rejected it as forced answer, nevertheless, one can argue that the answer of the Pnei Yehoshua himself is no less forced, to say that the Gemara is only asking that we should learn from sodeh that tomun is exempt, but for everything else no extra words are needed.
In addition one can argue that it is not so forced to say the Gemara is asking only according to R. Yehuda. Even though the halacha does not follow R. Yehuda, nevertheless see Biur HaGra Yoreh Deah 151:16 who writes that in several places the Gemara asks questions according to opinions which the Halacha does not follow. A similar principle is also stated in Teshuvas HaRashba 1:1199 and 1:1230 that in many places in the Talmud questions are asked which do not follow the Halacha. [An example is in Kesubos 16a where the Gemara asks that we should follow the majority of women who are besulos when they get married, even though we do not follow the majority in monetary matters - see Bava Kama 46b].
Good Shabbos
Dovid Bloom