Shevuot 42

In September 1999, a robotic probe launched by NASA to study the climate on Mars fell out of orbit and burned up in the Martian atmosphere. It was later discovered that the crash was caused by a mismatch between two onboard measurement systems, one of which used metric units and the other U.S. customary units. This simple error is estimated to have cost NASA hundreds of millions of dollars (U.S. dollars, to be precise). 

Rabbis, like rocket scientists and math teachers, appreciate the value of specifying units of measure. A mishnah on today’s daf teaches:

One takes an oath only concerning an item that is defined by size, by weight, or by number. How so? If the claimant says: “I transferred to you a house full of produce,” or: “I transferred to you a pouch full of money,” and the other person says: “I do not know how much you gave me, but what you left in my possession you may take,” and the amount in the house or pouch at that time is less than that claimed by the claimant, the defendant is exempt from taking an oath, as the amounts in the claim and the admission are undefined. 

Unless both parties are able to specify the amount of produce or money with recognized units — of which “pouchfull” is not one — the conditions that mandate the recitation of an oath are not met and an oath is not administered.

This mishnah is not, however, opposed to using newly invented units of measure.

If this party says that the house was full up to the ledge, and that party says that it was full up only to the window, the defendant is liable to take an oath.

If one cannot identify the quantity in broadly recognized units (for the rabbis, this would have been logs, kavs, se’ahs, etc.), then identifying it as filling the house to the point of a ledge or window is specific enough to require an oath. Even though these are not universal units of measurement, they are the basis for precise measurement — provided we have access to the house.

We know why NASA cares about units of measurement, but why does the Talmud in this situation? When the parties do not identify the exact quantity of what is owed, there is a greater possibility that the ensuing oath will be false — not due to intentional dishonesty, but rather because of the lack of specificity in the language. The rabbis are concerned about leading someone into making an accidentally false oath, because the infraction — violating one of the Ten Commandments — is so severe. Requiring all measures to be given in standard or at least specified units is one way to mitigate this risk.

The rabbis believed that, ultimately, the truth of an oath is not measured by the judges in rabbinic courts, but by God, whose name is invoked in each and every oath. The consequences of uttering a false oath are far more dire than losing a point on a math test or even an expensive satellite. To protect the defendant from incurring a divine punishment due to their lack of specificity, the rabbis exempt them, if they cannot provide the units, from having to take an oath at all.

Read all of Shevuot 42 on Sefaria.

This piece originally appeared in a My Jewish Learning Daf Yomi email newsletter sent on June 12, 2025. If you are interested in receiving the newsletter, sign up here.