Masechet Shavuot, Perek Zayin (Chapter Seven), Mishnah Gimel (Mishnah Three). The Mishnah says: Under what circumstances does somebody who was hurt swear and then collect damages? Witnesses testified that they saw a certain person enter another person’s domain, and that he was uninjured, whole. Then, when he walked out, he left injured. The witnesses saw him leaving with a new injury. The victim, the person who was hurt, said to the man whose place he had left, “You injured me.” The man replied, “No, I didn’t injure you; you hurt yourself.” The victim, the person who was injured, swears and then collects damages for his injury.
It’s pretty unlikely that a person would hurt himself. It’s likely that a person who was injured was injured by someone else. So Hachamim (Sages) made a decree and said that the victim is believed with a simple swear and can collect. The commentaries point out that he would only have to swear if the injury was on a part of his body where he could have done it to himself. For example, it was a bite on his arm. In that case, since it’s possible that the victim hurt himself, he has to swear to show that it wasn’t him. But if the injury was on a part of the body that it would be impossible for him to injure himself, for example, a bite on his back, a person can’t bite his back. So it’s obviously the person who entered his, who he was with, who did it to him. Therefore, in that case, according to Tanna Kama (first tanna), there isn’t even a need for anyone to swear.
But Rabbi Yehuda argues against the whole thing. Rabbi Yehuda, oh man, says that somebody who was injured cannot swear and then collect unless there is some sort of admission on the part of the person who caused the injury. What kind of situation? If the victim says to him, “You hurt me with two different injuries,” and the person replies, “No, I only gave you one injury.” In this case, where there’s some sort of admission, the victim would swear and receive compensation for both injuries. But if the other person denied the whole thing and said he never attacked him, then the victim cannot swear and collect.