Shiur Supplement - Daf 3b-4a
A. Ela atya me-eid echad... she-mechayvin oto shevu'a [lines 27-30] By this point the gemara has rejected the possibility of drawing a kal ve-chomer between one's own admission and the testimony of witnesses. We now compare the testimony of one witness to that of two in order to find support for the din of R. Chiya.
BACKGROUND: The gemara in Shevu'ot (daf 40a) learns out from a passuk that, although one cannot punish, fine or even obligate payment on the basis of one witness' testimony, nonetheless it is sufficient o obligate one to take a shevu'a. For example - Reuven claims 100 zuz from Levi who denies the claim entirely. Shimon, however, testifies on Reuven's behalf. We cannot obligate Levi to pay Reuven on the basis of Shimon's testimony, but we do require him to take an oath that he is, in fact, not beholden to Reuven.
The logic of the gemara is as follows: If ONE witness who CANNOT create a monetary obligation, can force the defendent to take a shevu'a, surely TWO witnesses who CAN create a monetary obligation, can force the defendant to take a shevu'a.
B. Ela amar R. Pappa ati mei-gilgul shevu'a de-eid echad [lines 2-3] BACKGROUND: The gemara (Kiddushin 27b) Learns out from the din of a Sota (suspected adulteress) that once a defendant ia required to take a specific shevu'a, we can require him to swear about other matters too. Even though, had the original oath not been in force, we would not have been able to require him to take the added shevu'a. [The Sota, by right, could be expected to swear only that she did not have sexual relations with the man her husband suspects. Nonetheless, she swears that she did not sleep with any man. This is expressed by the use of the double "amen."]