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, his word is accepted only from now, but not retroactively from the time that he specifies that he divorced her. That is, he is believed that he divorced her for all matters subsequent to this moment, 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, there is an obvious difference between the person's credibility in 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 (prohibited matters), such as whether the item became Tamei, one is believed by virtue of the fact that it was once in his ability to do to the object that which he says was done (see also RASHI to Gitin 54a, DH Rava). In contrast, in cases of monetary law, one is believed only when he presently has the ability to do what he says was done.
Why, then, in the case of the Gemara here, is the husband believed only with regard to the present time and onward, and not with regard to the past? The case of a Get is also a case of Isur (the Isur of Eshes Ish), and thus 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 such cases, so, too, a person is believed because of "b'Yado" even though it is no longer in his hands. The case of the Gemara here, however, involves the question of whether the woman was divorced or not, and thus it is a matter of a "Davar sheb'Ervah" in which a single witness is not believed. The credibility of testimony in a case of a "Davar sheb'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)