Before us is Mishnah 5 in the fourth chapter of Tractate Bava Metzia. The Mishnah itself is conceptually simple, but it appears complicated and confusing because it uses the names of coins that were current in the Mishnaic period. Therefore, we will explain it twice: first in a straightforward manner, using simple and familiar calculations, and afterward we will return to the numbers, the types of coins, and the currency used in the language of the Mishnah.
The Mishnah's Question:
"Kammah tehei hasela chaserah velo yehei bah ona'ah?" - How much can a sela be lacking without it constituting ona'ah? In our times, a coin only displays its face value, but in the Mishnaic period, coins were made of silver (and some of copper), and therefore they had intrinsic value. However, even if on the day it was minted the coin contained silver equal to its face value, over time it eroded: with each use a bit of metal wore off, and every day it oxidized. Everyone understands, therefore, that a coin does not necessarily contain the full value of the silver indicated by its denomination, yet it is still accepted as legal tender, since people do not trade with it by weight but rather as a coin - exactly as we do today.
It is worth noting that today no one expects silver or gold backing for the money we hold; this practice has been obsolete since the days of McKinley, and finally Nixon, who canceled it entirely in America. Nevertheless, we use money as usual. In the Mishnah as well, coins are used according to their face value.
However, there is a limit: if too little silver remains in the coin relative to its face value, trading with it becomes like a game of musical chairs. The coin passes from merchant to merchant, its intrinsic value continues to decrease, and eventually the music stops and one person is left holding coins with a face value of ten shekels, but which in reality contain only four shekels' worth of silver. Once people refuse to accept his coin, he will lose a great deal of money. Therefore, the Mishnah comes to teach: until when is it permissible to use a coin? What is the limit at which it has too little silver relative to its face value, meaning it can no longer be used, because it constitutes ona'ah and deceives people into accepting coins that are not worth their stated value?
The Three Opinions in the Mishnah:
"Rabbi Meir omer arba'ah issarin, issar ladinar" - Rabbi Meir says four issars, one issar per dinar. The measure is one twenty-fourth of the original value. If this were a coin of twenty-four shekels, as long as it is worth more than twenty-three shekels of silver, it is valid for use.
"Rabbi Yehudah omer arba'ah pundyonos, pundyon ladinar" - Rabbi Yehudah says four pundyonos, one pundyon per dinar. The measure is one-twelfth. For a coin of twelve shekels, as long as it retains silver worth more than eleven shekels, it is valid for use.
"Rabbi Shimon omer shemonah pundyonos, shenei pundyonos ladinar" - Rabbi Shimon says eight pundyonos, two pundyonos per dinar. The measure is a shtut (one-sixth), exactly like the measure of ona'ah in the previous Mishnah regarding other types of transactions. With coins as well, Rabbi Shimon holds that the measure is a shtut.
The Dispute Between Abaye and Rava:
In the Gemara, Rava and Abaye dispute whether this argument in our Mishnah regarding coins (a shtut, one-twelfth, and one twenty-fourth) applies to merchandise as well:
Rava: Indeed it does. Rabbi Shimon's opinion in our Mishnah, a shtut, is the exact same opinion we saw in the previous Mishnayos; whereas Rabbi Meir and Rabbi Yehudah hold that ona'ah on merchandise is also measured at one twenty-fourth or one-twelfth.
Abaye: Coins have a different rule. People expect a higher level of precision from a coin, since its value is fixed and known, and therefore Rabbi Meir and Rabbi Yehudah were stricter with it than with other types of merchandise.
The Halachah:
The Shulchan Aruch rules conclusively that the measure of ona'ah is a shtut, both for coins and for other merchandise. The Rema, on the other hand, takes the position that for coins the measure is one-twelfth, in accordance with the opinion of Rabbi Yehudah in our Mishnah, resulting in two different measures here - following the approach of Abaye. In practice, this has no halachic significance in our times, as our coins no longer function in this way.
The Coinage System in the Time of the Mishnah:
So far, we have reviewed the Mishnah in a nutshell. Now let us return to its text and detail the information needed to understand it:
Sela - The largest coin in the time of the Mishnah, the large silver coin. There are four dinars in one sela.
Ma'ah - The small silver coin. There are six ma'ahs in a dinar.
Pundyon - A copper coin. There are two pundyons in a ma'ah.
Issar - Smaller than it: there are two issars in a pundyon.
Perutah - The smallest of them all, which is not mentioned at all in our Mishnah: one-eighth of an issar.
According to this calculation, in terms of the issar: an issar - one; a pundyon - two issars; a ma'ah - four issars; a dinar - six ma'ahs, meaning twenty-four issars; a sela - four times a dinar, meaning ninety-six issars.
Calculating the Amounts:
Rabbi Meir - "Arba'ah issarin" - four issars: To the question "how much can the sela lack," meaning how much the large coin can lack and still remain fit for use, Rabbi Meir answers: up to four issars per sela. Four out of ninety-six is one twenty-fourth. And this is what the Mishnah says, "issar ladinar" - an issar to a dinar: in one dinar there are twenty-four issars, thus yielding one twenty-fourth.
Rabbi Yehudah - "Arba'ah pundyonos" - four pundyons: In a sela there are forty-eight pundyons, and four out of forty-eight is one-twelfth. Correspondingly, "pundyon ladinar" - a pundyon to a dinar: in a dinar there are twelve pundyons, which is again one-twelfth.
Rabbi Shimon - "Shemonah pundyonos" - eight pundyons: A lack of up to eight pundyons per sela. Since there are forty-eight pundyons in a sela, eight out of forty-eight is one-sixth. Correspondingly, "shnei pundyonos ladinar" - two pundyons to a dinar: in a dinar there are twelve pundyons, and two out of twelve is one-sixth.
In summary: In our Mishnah we learned how much a sela coin may erode without its use constituting ona'ah - one twenty-fourth according to Rabbi Meir, one-twelfth according to Rabbi Yehudah, and one-sixth according to Rabbi Shimon. We examined the dispute between Rava and Abaye whether these amounts were stated even regarding merchandise or if they were stringent only regarding coins, the ruling of the Shulchan Aruch (one-sixth for everything), and the opinion of the Rema (one-twelfth for coins). Finally, we detailed the coinage system - sela, dinar, ma'ah, pundyon, issar, and perutah - through which all these amounts become a simple calculation.