The Torah commands in Parashat Maseh (Bamidbar 35:30), “Ve’ed Ehad Lo Ya’aneh Be’nefesh La’mut” – “A single witness may not testify against a soul for death.” According to tradition, this verse establishes a prohibition forbidding a witness who testifies in a capital case from advancing an argument for the acquittal or conviction of the defendant. When dealing with a crime that is punishable by the death penalty, the witness is permitted only to provide testimony, to report what he saw, and is not allowed to voice arguments in favor of an acquittal or a conviction. Even if the witness is a Torah scholar who is well versed in the relevant laws, he is not permitted to give an opinion as to how the judges should rule. The reason behind this command is that the Torah treats human life with extreme care, and thus the death penalty may not be issued unless every precaution has been taken to ensure that the trial was fair and the ruling is correct. As such, the Torah does not allow the witnesses in such a case to say anything other than what they saw. Having witnessed the incident in question, they are, naturally, biased in one direction or another, and, as such, they are not entitled to voice an opinion. This applies only in cases involving a suspected capital offense. When it comes to monetary disputes, the witnesses are allowed to add their arguments in favor of the plaintiff or the defendant. However, even in monetary disputes, there is a rule “En Ed Na’asa Dayan,” which does not allow witnesses to serve as judges. Although witnesses in monetary cases are permitted to voice an opinion, they may not serve as judges who decide the case. This prohibition applies only to men, as only men serve as witnesses. A witness who testifies in a capital case and voices his opinion regarding the defendant’s guilt transgresses this prohibition, but he is not liable to Malkut, because this prohibition is transgressed without committing an action.