On the second side of today’s daf, we encounter this mishnah:
If the witnesses said, “this is our handwriting, however we were compelled to sign, or we were minors, or we were disqualified witnesses” — they are deemed credible.
However, if there are (other) witnesses who testify that it is their handwriting, or if their handwriting emerges on a document from another place — they are not deemed credible.
Suppose someone wishes to challenge a ketubah on the basis that the witnesses were invalid. According to this mishnah, if those original witnesses now testify that they were forced to sign against their will or were not qualified to sign (perhaps because they were minors at the time, or are relatives of the bride or groom) then we believe them. However, if other witnesses, either people or documents, can testify to the authenticity of the original ketubah, then the witnesses who wish to discredit their original signatures are not deemed credible.
But why? If we trusted the original witnesses to sign the ketubah the first time, why are they now no longer trusted to speak to their own signatures? Why do we favor the testimony of a second set of witnesses over the first?
We trust witnesses, the rabbis seem to be saying, except when they testify against themselves. In that case, we only trust them if we have no countervailing evidence.
Perhaps the reason is that allowing witnesses to testify against their original signatures in this way strips the ketubah of a great deal of its power. Want to divorce your wife and cheat her out of her ketubah payment? Call up your buddy who signed the ketubah and tell him to say he only signed under duress. Or that someone else forged his signature. A system that relies on witnesses is only good if we can, well, rely on those witnesses. The rabbis are therefore motivated to create a legal system in which a signature cannot be easily revoked. Part of that is cultivating a healthy suspicion toward those who come forward to say their original act of witnessing was invalid.
In a beraita (early teaching) quoted by the Gemara a few lines later, we see that Rabbi Meir was even more suspicious than his colleagues of witnesses who come forward to say that their own signatures are invalid. He discounts them in all cases:
Witnesses who testify to invalidate their signatures on a document are not deemed credible to invalidate the document — this is the statement of Rabbi Meir. And the rabbis say: They are deemed credible.
In the Gemara, Rav Hisda now explains Rabbi Meir’s thinking:
Rabbi Meir maintains: Witnesses who were told, “Sign a falsehood and you will not be killed,” should allow themselves to be killed.
If Rav Hisda is right, Rabbi Meir believed that bearing false witness is such a terrible crime, one should actually submit to death rather than commit it. Therefore, according to Rav Hisda’s logic, Rabbi Meir also believed we cannot accept later testimony from those witnesses that their original signatures were invalid. That’s how important witnesses are: The Jewish legal system really needs them to be trustworthy.
If Rabbi Meir’s position, especially as explained by Rav Hisda, seems too strident to you, you’re not alone. The rabbis thought so too. Famously, there are just three sins that the Talmud believes one should submit to death rather than commit: murder, idolatry and adultery. Bearing false witness is not on the list (though it does make the Ten Commandments). The Gemara therefore searches for another explanation for Rabbi Meir’s position. But his basic point is clear: We really need to trust witnesses. And therefore we do — when they’re signing.
Read all of Ketubot 18 on Sefaria.