Question

Tosafot (in Gittin 2b, s.v. “and if it has upon it”) say that we certainly make the claim that it may be forged, because otherwise you would leave no one able to live. But I didn’t understand: this seems to contradict an explicit Talmudic statement that witnesses who sign on a document are regarded as though their testimony had already been examined in a religious court, and the reason is that a person is not so brazen as to forge it. This seems to contradict Tosafot: on the one hand they say that ordinary people do not forge, and on the other hand they say, “otherwise you would leave no one able to live”?!