From our experiences with secular courts, we often mistakenly think that witnesses who come to testify in court must swear to tell the truth before offering their testimony. The Torah never requires witnesses to take an oath – the court either accepts the witness as being reliable, or else distrusts him based on his connection to the case or because of questions as to his reliability.
The Torah, does, however, occasionally require the defendant in a case to swear that the version of events he/she presents is true. One situation that is based entirely on the defendant swearing that she is telling the truth is the case of sota – a woman whose husband suspects her of being unfaithful. In such a case – which is described in detail in 5:11-31 – if there are witnesses, then the woman would be tried as an unfaithful wife. It is only if there are no witnesses that the procedure described in the Torah is carried out, culminating with the shevu’a – the oath – recited by the kohen and accepted by the woman with her statement of “amen, amen” (Bamidbar 5:22).
Our Gemara quotes a Mishna that teaches how the woman’s oath that she did not commit adultery applies not only to suspicions raised following her marriage (nissuin), but also after kiddushin (betrothal) and even during the period that she was a widow waiting for yibum (levirate marriage).
After a lengthy discussion of the mechanism that would allow the oath to cover all of these different periods, Rav Nahman bar Yitzhak suggests that it works through gilgul shevu’a, extending her oath. The idea of gilgul shevu’a is that once someone is obligated to swear on a given issue, we also require them to take a stand on other, connected questions, even though ordinarily they would not have required an oath. Gilgul shevu’a is a concept that appears often in monetary matters, and Rashi on our daf suggests that it is the case of sota that serves as the primary source for this idea.