Beitzah 13B

אלא מן הגורן ומן היקב, כך תרומת מעשר אינה ניטלת אלא מן הגורן ומן היקב.

but only from the granary and the winepress, so too, teruma of the tithe is separated only from the granary and the winepress.

מחשב? הא מדידה בעי!

The Talmud asks: If the Mishnah is referring to produce from which teruma gedola has not been separated, it is appropriate to use the term: Calculates. However, according to the suggestion that it is referring to a first tithe, from which teruma of the tithe must be separated, why does the Mishnah state: Calculates? The tanna of the Mishnah should have said: Measures. The amount of teruma gedola to be separated is calculated by estimation, as there is no fixed amount for this teruma according to Torah law. With regard to the teruma separated from first tithe, however, the Torah established the fixed amount of 1/10th, and one is required to measure precisely.

הא מני אבא אלעזר בן גימל היא, דתניא: אבא אלעזר בן גימל אומר: ״ונחשב לכם תרומתכם״⁠ — בשתי תרומות הכתוב מדבר, אחת תרומה גדולה ואחת תרומת מעשר. כשם שתרומה גדולה ניטלת באומד ובמחשבה ⁠— כך תרומת מעשר ניטלת באומד ובמחשבה.

The Talmud explains: In accordance with whose opinion is this Mishnah? It is in accordance with the opinion of Abba Elazar ben Gimmel, as it is taught in a baraita: Abba Elazar ben Gimmel says: The verse states: “And your teruma should be reckoned to you as though it were the corn of the granary and as the fullness of the winepress” (Numbers 18:27). The verse speaks of two terumot. One is teruma gedola, and the other one is teruma of the tithe. Just as teruma gedola is separated by estimation and is not measured exactly, and it is enough to separate it by thought, as the word “reckoned” implies that the mere intention to separate a particular portion serves to remove the rest of the produce from its untithed state, so too, teruma of the tithe can also be separated by estimation and by thought.