A question on the Mishna, Negaim 11:7

Question

In Negaim 11:5, the mishna lists one of the preconditions for burning a garment: if its green or red discolouration hasn't changed for two weeks. Two mishnayot later, in 11:7, Rabbi Eliezer is asked why a garment that doesn't have room for the discolouration to spread would still need monitoring, and he doesn't know the answer. Rabbi Yehuda ben Beteyra tells him that it is because the discolouration might remain for two weeks, in which case the garment would need to be burnt, and Rabbi Eliezer praises him for being a great scholar (חכם גדול אתה).

What is so impressive about Rabbi Yehuda ben Beteyra's answer? Did Rabbi Eliezer not know the law recorded in Negaim 11:5?? I find that rather unlikely, since it's also recorded in Leviticus 13:55. Seeing as it is recorded there (and therefore not a particularly profound observation), why did Rabbi Eliezer not say it himself, and why was he so impressed when Rabbi Yehuda ben Beteyra did?

Answer 1

I think the P'Shat here is that R' Yehuda ben Beseira is being praised for strengthening and substantiating the words of the Chachomim, not for the wisdom of the actual reason. There is a similar exchange between these Tannaim in 9:3 and there, the Tosfos Yom Tov explains the praise Chachom Gadol. "You are a Chachom Gadol because you supported the words of the Sages by giving a reason for them and that is better than contradicting them." He also quotes the Korban Aharon that this praise was earned because he did not confuse the question and contradict their teachers.

Answer 2

Not sure but it seems that after Mishna 5 you can believe that the Hesger, the first hesger, is practicable only if there is a surface size enough for potential spreading. In the verse also.we can get a such understanding. The Chiddush ( novelty) is that we have a reason to make first Hesger, even if the only possibility which can lead the cloth to uncleanness is an eventual second Hesger ( and not an enlargement of the leper) and after this second Hesger ... the remaining of the leper in the same state makes the uncleanness.

וְ֠הִנֵּ֠ה לֹֽא-הָפַ֨ךְ הַנֶּ֤גַע אֶת-עֵינוֹ֙ וְהַנֶּ֣גַע לֹֽא-פָשָׂ֔ה טָמֵ֣א ה֔וּא ‏

Rabbi Yehuda Ben Betera provided this semantic novelty in the verse.

Summary:

1. There is an Hava-Amina (a step in which we were possibly thinking) to reject the possibility of Hesger Rishon because of the lack of potential spreading.
2. The conclusion is the profound insight of Rabbi Yehuda in the lecture taught by the wises for the reason of making a first Hesger, even if the only way to reach Tum'a is the second Hesger.

I hope that this makes sense.