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Why did the rabbis of the Talmud use approximate weights and measures, such as k'zayit, k'betzah, revi'it, k'dei achilat pras, amah, etc. [1], which caused disagreements among later Sages as to what they meant, when precise units were readily available in Greek and Roman times [2]?

Some say they wanted to relate the quantities to everyday matters (olives, eggs, loaves of bread, etc.) so the average Jew would understand. But they could have used BOTH. Surely they were aware that later generations would have a problem understanding precisely. And sometimes they DID use precise Roman measures.

The only logical conclusion is that they did not want to set the matter in stone, but INTENDED it to be approximate. (Of course, they did not anticipate that genetic manipulations could produce giant olives, but that's another matter. :-) )

[1] See http://halachipedia.com/index.php?title=Reference_of_Measurements_in_Halacha
[2] See https://en.wikipedia.org/wiki/Ancient_Roman_units_of_measurement

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    Perhaps the main reason is that with a huge concern of exile, the rabbis may have been worried that Roman measurements would be lost or not used in the places were Jews would eventually live. Indeed, do you know any of the Roman measurements, yourself? Living in the U.S., I have some trouble understanding metric units. Relating things to common objects is a bit more portable and lasting. It's also not uncommon even in current society. The U.S. Weather service measures hailstones by comparing the size to objects such as quarters and grapefruits. – DanF Feb 1 '18 at 15:49
  • Maybe measuring tapes and scales weren't so common, while everyone had seen an egg, an olive and an arm-length. – mbloch Feb 1 '18 at 15:52
  • Many measurements are from the Torah, not the Rabbis. – Heshy Feb 1 '18 at 22:46
  • @Heshy but the Rabbis could have retold the measurements in their terms (like instead of saying "1 Reviyis", say" 86ml). – Shmuel Brin Feb 2 '18 at 3:24
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The Rabbis of the Talmud used measurements of olives etc. because those are the measurements used by G-d when he gave them - as the Talmud says in sukkah, ארץ זית שמן - ארץ שכל שיעורה כזיתים, "the land of oil-giving olives" - the land whose measurements are based on olives.

While one might ask why they didn't translate that into other, equivalent, measures, which may be more easily acceptable or standard, the true answer is that they did - every measurement is the Talmud has a formula, given in the Talmud, for how to translate it into any other measure.

Uncertainty only began when people were unsure what one of these equatable measures was the base, and therefore when we have multiple sample sizes of several (eg. both eggs and olives), which one should be the base unit to determine all of the others, complicated by the fact that the size of people changed over the centuries and millennia, particularly when diets changed, making the task of linking these to the size of average body parts in the time of the Talmud (or nowadays) far harder.

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You ask about "k'zayit, k'betzah, revi'it, k'dei achilat pras, amah, etc", i.e. lengths and volumes. The Wikipedia article on which you base your premise "precise units were readily available in Greek and Roman times" starts its discussion of measures of volume thus:

Both liquid and dry measures were based on the sextarius. As no two surviving examples are identical, scholarly opinion ranges

It mentions similar ambiguity about lengths. So there's no reason to assume a less ambiguous method existed in those days.

  • Additionally, the Wikipedia article mentions fingers and feet. That seems intensely inaccurate, and they didn't have Wilt Chamberlain at the time. – DanF Feb 1 '18 at 22:09
  • @msh210 -- the range of uncertainty is larger for talmudic units than for Roman units. – Maurice Mizrahi Feb 1 '18 at 23:47
  • @MauriceMizrahi The range of uncertainty today with Talmudic units doesn't mean there was as much uncertainty back then. If the only evidence for Greek/Roman measurements were textual, I'm sure they would also be a lot less certain. Many of the measurements are actually exactly the same in name (e.g. אמה=arm, זרת=finger) and would therefore face the same uncertainty – b a Feb 2 '18 at 11:14
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This question was posed to R. Sherira Gaon and he answered that the Sages gave the measurements in terms of fruits and the like because those are constantly around and we can't argue that they have changed. The measurement is meant to be determined by the assessment of the the individual person conducting the examination.

Responsa and Explanations of R. Sherira Gaon Vol. 1 #60 תשובות ופירושי רב שרירא גאון א:ס

לרב שרירא וששאלתם לפרש לכם אם יש משקל לגרוגרת ולזית ולכותבת ושאר שיעורים במשקל כספים של ערביים ופירשתם שמר רב הילאי גאון ז"ל פירש לכם משקל ביצה ששה עשר כסף ושני שלישי כסף ואם אין לשאר משקל מאי טעמא ביצה יש לה משקל הוו יודעים שאין לאלו שעורים משקל כסף לא במשנה ולא בתלמוד שלנו כל עיקר ואלו בקשו לתת שעור משקל מן הדינרים היו עושין מעיקרא אלא שנתנו השיעור מקטנית ופירות שמצויין תמיד ואין לומר שנשתנו וכך שנינו כביצה [שאמרו] לא גדולה ולא קטנה אלא בינונית ר' יהודה אומר מביא הגדולה שבגדולות והקטנה שבקטנות [ונותן לתוך המים] וחולק את המים אמר ר' יוסי מי מודיעני איזה גדולה ואיזה קטנה אלא הכל לפי דעתו של רואה ומפורש בתוספתא משים כוס מלא מים ומביא אוכלין שאינן בולעין ונותן לתוכן עד שיחזרו המים לכמות שהיו וחוזר וחולקן הא למדנו משנה שלימה [שאין לביצה] משקל ידוע שלא להוסיף ושלא לגרוע וסלקא דעתין ר' יהודה ור' יוסי דקאמרי האיך משערים בביצה לא היו יודעין לקוץ לה משקל ידוע וכי מר רב הילאי גדול וחכם מר' יהודה ור' יוסי היה אלא כמתני' עבדינן הכל לפי דעתו של רואה ומר רב הילאי הכין הות דעתיה שהמשנה חייבה כל אדם לפי דעתו והרי פירש לכם מה שראה הוא לפי דעתו או מה שראו משלפניו ולמדוהו אם חפצים אתם לעשות כמוהו ולסמוך עליו או על חכמים שלפניו ששערו כן לפי דעתם עשו ואם לאו שערו אתם ולפי דעתו של רואה יעשה אם חסר אם יותר וכן לענין כזית וכגרוגרת הרי פורש לכל אחד מהן בזו משנתינו לא גדול ולא קטן אלא בינוני וגם הוא לפי דעתו של רואה והאי דקצת רבנן שערו לפי דעתם ביצה ולא שערו כזית וככותבת וכגרוגרת משום דביצה תלויין בה מילי נפישי קב וסאה ואיפה ועומר כולהון משתערין בביצה לפיכך שערוה לפי דעתם אבל הני שיעורים שבקינון לפי דעתו של רואה

  • So why is there an argument over the size of a Koseves Hagassah in Yomah? – Shmuel Brin Feb 2 '18 at 3:25
  • Someone show this to... Was it R’ Yehuda HaChassid who wrote the Teshuvah about the size of a beitzah? – DonielF Feb 2 '18 at 3:40
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My line of reasoning follows the ruling of the New Moon - the Hebrew Rosh Chodesh. The contradiction lies in the very sentence the Halakhah is derived from - "החודש הזה לכם" - juxtaposing "הזה" ("exactly like this") to "לכם" ("as you wish").

In other words, the argument whether the truth of the Torah is in the Heaven (and hence "objective") or on the Earth (and hence "subjective") is rooted in the Torah itself (Deut 30,12 and B"M 49b). The Gemmorah (ibid and all over the Shas) brings numerous occasions on which the sages argued whether the original truth must be discovered and followed or the sages can decide according to their needs. This is especially true for measures.

An interesting point is the fact that the Torah seemingly obligates to get disputes resolved by the Sanhedrin (which was active through the times of Sages), however they never followed it, which clearly (for me) indicates that they adopted the second principle of "subjectivity" and setting (each one) their own standards, just like R' Yehosuah set his own Rosh Chodesh.

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