If a man reports to the beit din (the Jewish court) that a certain man has died, based on that testimony the beit din will act to allow the dead man’s wife to marry. The Mishna on our daf teaches that if the man testified that a man had died, or if he said that he had killed a certain person, or that he was involved in the murder, the beit din will accept his testimony and permit the wife to marry. Nevertheless, the witness (or, perhaps, the murderer) will not be allowed to marry the widow himself. Rabbi Yehuda disagrees in the case where the man testifies that he was the murderer, and says that in such a case we cannot accept his testimony at all, since we do not allow a person to incriminate himself. Therefore the woman cannot marry him or anyone else, since we must assume that her husband is still alive.
According to the Gemara, the Tanna Kamma of the Mishna also agrees that we cannot allow a person to incriminate themselves. The opinion is presented in the name of Rava that adam karov etzel atzmo, ve-ein adam masim atzmo rasha – just as a person cannot testify against a close relative in beit din similarly he cannot testify against himself, incriminating himself. Apparently, however, the Tanna Kamma relies on an often discussed Talmudic idea – palginan diburei – we split up his statement. In this case that means that we reject his self-incriminating statement, but we accept his testimony that the man had actually been killed.
The mechanism behind the concept of palginan diburei is subject to a disagreement among the rishonim. The Rashba argues that we can only apply it in cases where we can interpret the testimony in a way that will allow his entire statement to be understood as being truthful. For example, in our case, we could say that the witness who says “I killed him” actually means “I killed him…accidentally.” If it is impossible to interpret his testimony in such a way, we would not apply the principle of palginan diburei, and we would reject his testimony entirely. Others, however, explain that the concept of palginan diburei is powerful enough to allow us to accept the conclusion of his testimony (that the man is dead) even as we reject the incriminating aspect of it (that the witness murdered him).