Sukkah 36A

הא בכולה, הא במקצתה.

this Mishnah, where it states that if the etrog was peeled it is unfit, is in a case where all of it was peeled. That statement of Rava that if it was peeled it is fit is in a case where only part of it was peeled.

נסדק, ניקב. תני עולא בר חנינא: ניקב נקב מפולש ⁠— במשהו, ושאינו מפולש ⁠— בכאיסר.

The Mishnah continues discussing the halakha of an etrog that was split or pierced. Ulla bar Ḥanina taught: An etrog that was pierced with a hole that completely goes through its body is unfit with any size hole. If the hole does not completely go through the etrog, it is unfit only with a hole the size of an issar coin.

בעי רבא: נולדו באתרוג סימני טרפה, מהו? מאי קמיבעיא ליה? אי נקלף ⁠— תנינא! אי נסדק ⁠— תנינא! אי ניקב ⁠— תנינא!

Rava raised a dilemma: If signs of a tereifa developed in the etrog, what is its halakhic status? The Talmud clarifies: What is the dilemma that he is raising? There are similarities between the halakhot of the etrog in the Mishnah and some of the halakhot of a tereifa, a bird or animal with a condition that will lead to its death within a year. If it is the case where the etrog was peeled, we already learned that case. If it is the case where the etrog was split, we learned that case as well. And if it is the case where the etrog was pierced, we learned that too. After ruling out those defects, the question remains: With regard to what is Rava’s dilemma?

כי קא מיבעיא ליה כדעולא אמר רבי יוחנן: ריאה שנשפכה כקיתון ⁠— כשרה. ואמר רבא: והוא דקיימי סימפונהא. הא לא קיימי סימפונהא ⁠— טרפה. הכא מאי? דלמא התם הוא דלא שליט בה אוירא, הדר בריא, אבל הכא דשליט בה אוירא ⁠— סרוחי מסרחא, או דלמא לא שנא?

The Talmud answers: When he raises the dilemma, it is with regard to a case like that which Ulla said that R' Yoḥanan said: A lung whose contents can be poured like a pitcher, i.e., whose tissue dissolved to the point of liquefaction, is not a sign of tereifa, and the animal is kosher. And Rava said: And that is the halakha only where the bronchia are intact. However, if the bronchia are not intact, it is a sign of tereifa. The dilemma here is with regard to a comparable situation in an etrog, i.e., an etrog that liquefied from within: What is its halakhic status? Perhaps it is there, in the case of the lung, where the air does not affect it since it is completely enclosed in the body, that the lungs can recover, and that is why it is not a tereifa. However, here, in the case of the etrog, where the air affects it, it inevitably decays and spoils and therefore it is a tereifa. Or, perhaps the case of the etrog is no different.