QUESTION: Aba Elazar ben Gamla states that from the verse's comparison of the laws of Terumah Gedolah with the laws of Terumas Ma'aser, we derive that just as Terumah Gedolah may be separated by estimating the amount (one-fiftieth) which one must separate, so, too, Terumas Ma'aser (one-tenth) may be separated by estimate , and one is not required to make an exact measurement.

RASHI explains that the one-fiftieth of one's produce which the Rabanan required to be separated as Terumah Gedolah may be separated based on an estimate, because even if one mistakenly separates too little he still fulfills the Torah requirement. The Torah requires that only a single grain of wheat be separated.

Aba Elazar ben Gamla's teaching is difficult to understand. He derives from the verse the Torah law that one may rely on an estimate when one separates Terumah Gedolah. However, if, as Rashi explains, mid'Oraisa even one grain of wheat suffices for Terumah Gedolah, there is no basis for estimating the Shi'ur of Terumah Gedolah, as there is no minimum Shi'ur that must be estimated! Why does he say that "just as the Torah permits estimating for Terumah Gedolah, it permits estimating for Terumas Ma'aser"? Terumah Gedolah has no Shi'ur, and thus estimating is irrelevant, while Terumas Ma'aser, in contrast, has a minimum Shi'ur mid'Oraisa.

ANSWER: Rashi seems to understand that mid'Oraisa, l'Chatchilah a person must separate a certain amount of his produce as Terumah, and it does not suffice to separate just one grain l'Chatchilah. Only b'Di'eved does a single grain exempt the rest of the produce. It is not logical that the Torah would permit one to separate a single grain l'Chatchilah as Terumah, because the purpose of Terumah is to provide sustenance for the Kohanim; if everyone would give the Kohanim only one grain, they would receive no significant form of sustenance. It must be that the Torah requires that a larger Shi'ur be separated l'Chatchilah, but since the Torah does not specify that Shi'ur, it must be that the Torah empowered the Chachamim with the authority to determine an appropriate Shi'ur. The Shi'ur which the Chachamim establish will be considered a Shi'ur mid'Oraisa. (See Chagigah 18a; RAN to Yoma 73a; TOSFOS to Yoma 79b, DH Lomar Lach.) Therefore, although it was the Chachamim who established that the Shi'ur of Terumah is one-fiftieth, that Shi'ur is considered a Shi'ur mid'Oraisa.

(TOSFOS and the TOSFOS HA'ROSH also question the usage of the word "estimate" with regard to Terumah Gedolah. They suggest that the word "estimate" does not apply to Terumah but only to Terumas Ma'aser, and Aba Elazar is merely deriving that just as one may add as much as he wants to Terumah Gedolah, he may add as much as he wants to Terumas Ma'aser. Rashi avoids their question by explaining that there is a Shi'ur mid'Oraisa for Terumah Gedolah, which one must separate l'Chatchilah.)