QUESTION: The Gemara teaches that a husband is believed to say that he
divorced his wife (since it is in his hands to divorce her now). However, we
only accept his word from now, but not retroactively from the time that
specified that he divorced her. For all subsequent matters, he is believed
that he divorced her, but he is not believed that he divorced her with
reference to the past. This is because his credibility is based on his
ability to give her a Get now -- "b'Yado." He cannot give her a Get to take
effect in the past, and therefore his credibility does not extend to the
past.
A similar concept is expressed by the Gemara in Gitin (54b). If a person was
kneading dough and upon finishing he told the owner of the dough that it
became Tamei, he is believed. He is believed because it was in his ability
to make the dough Tamei while he was kneading it.
However, we see that there is an obvious difference between the person's
credibility in each of these two cases. In the case in Gitin, the person
kneading the dough is believed to say that the dough became Tamei even
though the dough is no longer in his hands. Rava there explains that in
questions of Isur (matters of prohibitions), such as whether the item became
Tamei, as opposed to cases of monetary law, one is believed by virtue of the
fact that it was *once* in his hands to do to the object that which he says
was done (see also RASHI to Gitin 54a, DH Rava).
Why, then, in the case of our Gemara, is the husband believed only with
regard to the present time and onward, and not with regard to the past? This
case is also a case of Isur (the Isur of Eshes Ish), and the husband should
be believed with regard to the past as well, since his credibility is based
on "b'Yado!"
ANSWER: The SHEV SHEMAITSA (end of 6:1) explains that just as there is a
leniency in cases of Isur in that one witness is believed in cases of Isur,
so, too, a person is believed because of "b'Yado" even though it is no
longer in his hands. In our Gemara, where the issue is whether the woman was
divorced or not, the case is one of a "Davar she'b'Ervah" in which a single
witness is *not* believed. The credibility of testimony in cases of "Davar
she'b'Ervah" must be of a higher degree than that which is required for
normal cases of Isur. Hence, a proper "Migu" is required, and for the
husband to have a proper "Migu" it must be in his hands to divorce his wife
right now. He is not believed about the past based on a "Migu" that he
*could* have divorced her at that time, because that would be a "Migu
l'Mafre'a" which does not work (see TOSFOS to Bava Basra 30a). (Y. Marcus)