Beyond that, of course, there is no law of informer at all regarding testimony before a state court. In my view, that is a rather simplistic and foolish notion.

Hello Rabbi Michael,
With your permission, I’d like to share the following thought with you:

Assuming that a religious court may punish not according to Torah law, and that according to Rashba they have the authority to accept invalid testimony such as that of a relative, a woman, or a single witness,
and further assuming that the reason for disqualification in the case of an interested party is suspicion of lying (and not because a person is considered like his own relative—in which case the problem could be solved through the court’s authority as mentioned),
and assuming that there is no supporting evidence for the state’s testimony, meaning this is only his own testimony (and therefore it is impossible to verify positively that he is not lying),
a question arises whether the court’s said authority is really so sweeping that it can ground a conviction on the basis of a witness suspected of lying. After all, even Rashba did not refer to witnesses who are personally disqualified because of lying, but rather to a minor, a woman, a relative, and a single witness.
In other words: the scope of the court’s authority to accept invalid evidence is not yet fully clear to me, and seemingly, insofar as the state’s witness is an interested party because of suspicion of lying and there is no additional evidence—I do not know whether or how his testimony can be validated.

Thank you very much

Hello N.,
Suspicion of lying has two aspects that seem opposite, but are really two sides of the same coin: on the one hand, if the suspicion truly exists, there is no way to accept his testimony by formal means (this is what the medieval authorities wrote regarding a robber in testimony to permit a woman to remarry, where they validated all invalid testimonies but not those of people suspected of lying). On the other hand, if in a particular case a situation arises where the suspicion of lying does not exist, then there is no obstacle at all to accepting his testimony. Therefore, with disqualifications due to suspicion of lying, everything depends on the question whether there is actually suspicion of lying in this specific case, and there is no significance to statements by the medieval authorities or even by the Talmud itself that determine that a person of this or that type is or is not suspected of lying. It depends on the judges’ assessment of reality in the specific case before us. Thus, for example, many halakhic decisors accepted Sabbath desecrators as witnesses, because nowadays they are not suspected of lying (they are not value-driven criminals; they simply do not believe and are not committed to Jewish law).
As far as I know, nobody even considers accepting the testimony of a state’s witness without independent corroboration. Every child understands the biases and suspicions involved in such testimony.
Beyond all this, of course, the legal institutions of the state are not at all troubled by the determinations of Jewish law, so the discussion is not really practical, obviously. On the other hand, they themselves will not accept testimony that in their eyes is suspected of being false, even if they are not bound by Jewish law. Therefore, the assumption is that if they decided to accept the testimony, they assess that there is no suspicion of lying—and in such a situation, from the standpoint of Jewish law as well, there is no obstacle to accepting the testimony.

Thank you very much for the detailed answer.
My question is not directed at the institutions of the state at all.
And in the end—if it can be clarified in one way or another that he did not lie, then it can be accepted by virtue of the court’s authority to depart from the ordinary rules of evidence.
And if there is no supporting evidence whatsoever for his version from an independent source that would make clarification possible, and the built-in suspicion of falsehood has not been removed—then presumably the testimony cannot be accepted.
In the final analysis, it is like Israeli law—a state’s witness is suspected of lying, and therefore the law requires, under section 54A of the Evidence Ordinance, corroborative evidence, which is strong evidence from an independent source. Without it, one cannot convict.
Thank you for your comments—they helped organize my thinking further.
Thank you very much

Exactly—that is what I wrote, and therefore I argue that both in Jewish law and in the law, this is simply common sense. When there is suspicion of lying, the testimony is not accepted; and when there is corroboration that removes the suspicion, it is accepted.