Bava Metzia 4A

תאמר בעדים, שעל מה שכפר הוא נשבע.

Would you say the same with regard to two witnesses, in which case the defendant takes an oath with regard to the claim that he denies, and not with regard to the debt about which they testify?

אלא אמר רב פפא: אתי מגלגול שבועה דעד אחד.

Rather, Rav Pappa said: R' Ḥiyya’s a fortiori inference is derived from the extension of an oath that the testimony of one witness obligates him to take. Once a defendant is obligated to take an oath, the plaintiff can demand that he take an oath with regard to other claims that he has against him as well.

מה לגלגול שבועה דעד אחד, שכן שבועה גוררת שבועה, תאמר בעדים ⁠– דממון קא מחייבי!

The Talmud rejects this: What is notable about the extension of an oath that is obligated by the testimony of one witness? It is notable in that one oath leads to another oath. The obligation to take the second oath is not caused directly by the witness. Would you say the same in the case of witnesses, whose testimony renders the defendant liable to pay money? Incurring liability to pay one part of the claim does not extend further and incur liability to pay the rest.

פיו יוכיח. מה לפיו שכן אינו בהכחשה ⁠– עד אחד יוכיח שישנו בהכחשה ומחייבו שבועה.

The Talmud responds: The admission of one’s mouth can prove that an element other than an oath can cause the defendant to be obligated to take an oath with regard to the rest of the claim. Although the a fortiori inference from admission alone was already rejected, due to the claim of: What is notable about the admission of one’s mouth, it is notable in that it is not subject to contradiction; the testimony of one witness can prove that even testimony that is subject to contradiction renders one liable to take an oath.