Sukkah 35 - Etrog, or citron
If the etrog is the priest's portion (terumah) and is ritually impure - it is invalid because one cannot eat it. If it is the priest's portion and it is pure - then one should not take it so that he does not make it impure, but if he does - it is valid.
But how do we know that "etrog" is citron in the first place? Since the Torah calls it "beautiful tree-fruit," we understand that it comes from such a special tree whose branches also have the taste of its fruit - and that is citron. But perhaps it is pepper? - No, that could not be: if you take just one pepper, it is too small. And if you take two or three - the Torah said to take one fruit, not two or three. Thus it must be citron.
Art: Pepper And Lemon On A White Tablecloth By Odilon Redon
