Kiddushin 64B

דמוחזק לן באחי ולא מוחזק לן בבני, דאמרינן: ״מה לי לשקר״: מאי קאמר, למיפטרה מיבם? מצי אמר: פטרנא לך בגיטא.

a case where we have a presumptive status that he has brothers, but we do not have a presumptive status that he has children, although no full-fledged testimony has been submitted for either claim. As in that case we say and apply the logic of: Why would I lie? In other words, if he were lying he would have stated a more credible claim or performed an action achieving the desired result. The Talmud explains: For what reason is he saying that he has children or that he has no brothers, if not to exempt her from a yavam? But if so, he can say to her instead: I exempt you by means of a bill of divorce. Consequently, it can be assumed that he is telling the truth when he says that he has children.

רבי סבר: ״מה לי לשקר״ כי עדים דמי, ואתו עדים עקרי חזקה. ורבי נתן סבר: ״מה לי לשקר״ כי חזקה דמי, לא אתיא חזקה ועקרה חזקה לגמרי.

In light of the above explanation, the Talmud clarifies the dispute in the baraita. R' Yehuda HaNasi maintains that the claim of: Why would I lie, is considered like the testimony of witnesses, and witnesses can come and uproot a presumption. Therefore, he does not have the power to contradict his previous claim and say that he has no children and render her forbidden at the time of his death. And R' Natan maintains that: Why would I lie, is considered like a presumption, and a presumption cannot come and uproot another presumption entirely. As a result, he is deemed credible even for the purpose of rendering her forbidden, i.e., if he says at the time of his death that he has no children, the earlier presumption that he had brothers, but no children, is reinstated.