The Lone Witness

In regard to monetary laws the Torah requires two witnesses to take money away from someone, however, one witness is believed to administer an oath to the party contradicting his testimony.  The Gemorah says in many places (Gittin 2b, Yevamot 98a etc.) that one witness is believed regarding issurin.  The opinion of Tosfos (Gittin 2b) is that the source for the rule that one witness is believed regarding issurin is derived from וספרה לה (טו:כח), which the Gemorah (Ketubot 72b) learns from there that a woman is believed to say if she is a niddah or not. 

There are two ways one can understand the difference between monetary laws and issur.  It can be that the witness is believed regarding issur just as two witnesses are believed in regard to monetary laws, regarding issur one witness suffices.  Another possibility is that issur doesn’t have any connept of testimony, its just a question of establishing if something is muttar or assur, hence there is נאמנות to the witness just as other forms of בירורים would function to determine the status of the issur; in other words is the עד believed as נאמנות או עדות.

The Tashbatz (siman 77) says that it is considered a to be עדות (he says one witness isn’t believed regarding monetary laws for there is a slight chance he is lying but we aren’t worried about that slight possibility for issur.)  On the other hand, the Taz and Chavvas Daas in the beginning of Yoreh Daeh siman 98 say that testimony on issur isn’t considered עדות and its just a נאמנות.  The Shach 127:14 also holds that its not עדות, he even entertains that one witness may only be believed because of a תקנה.

This question would seem to be the debate if we apply the principle of כל מקום שהאמינה ע"א ה"ה כב' regarding a single witness on issur or the principle doesn’t apply here (see Shach ibid.)