Shevuos, Chapter 7, Mishnah 3. The Mishnah adds another of the cases listed in the first Mishnah, in which the claimant is the one who takes an oath and collects.
"HaNechbal keitzad?": - The case of one who was injured, how so?
"Hayu me'idim oso shenichnas tachas yado shalem v'yatza chaval" - witnesses testify that they saw the injured man enter another person's premises whole and healthy, and when he came out he had a wound on his body.
"V'amar lo chavalta bi, v'hu omer lo chavalti" - the injured man claims against the owner that he is the one who wounded him, and the owner responds that he did not wound him at all, but rather the injured man did it to himself.
"Harei zeh nishba v'notel" - the injured man takes an oath supporting his claim, and collects the value of his damages from the defendant.
The Rishonim disagree about the reason for this ruling:
The Rambam: this is a penalty and a punishment imposed on the one who wounded, as explained above, since we do not want people to go about injuring one another.
The Raavad: this is not a penalty at all, but a compelling logical argument: people do not normally wound themselves, and therefore the defendant's claim of "you injured yourself" is not plausible. For that reason he is not believed, and we place the oath upon the injured party, who takes the oath and collects.
And if there is clear and certain proof that it was his fellow who injured him and that he did not injure himself - for example, a bite was found between his shoulders, in a place where a person cannot possibly bite himself, and there was no other person there apart from the defendant - he collects even without any oath at all, since there is no other possibility besides the defendant having inflicted the wound.
The view of Rabbi Yehudah:
Rabbi Yehudah adds here the position he took in the other cases as well: "ad shetehei sham miktzas hoda'ah" - we do not transfer the oath and place it upon the injured party unless there is a partial admission. How so? The injured man claims "chavalta bi shtayim" - you wounded me twice, and the defendant responds "lo chavalti becha ela achas" - I inflicted only one wound, and the second one you inflicted on yourself. Since there is a partial admission here and the defendant cannot take an oath, the injured party is the one who takes an oath and collects.
In summary: in this Mishnah we learned the law of one who was injured, that when witnesses testify that he entered whole and came out wounded, he takes an oath and collects; the Rambam and the Raavad disagree about the reason for the ruling, the Rambam explaining that it is a penalty imposed on the one who wounded, and the Raavad explaining that the defendant's claim is not plausible. We further learned that where the proof is certain he collects even without an oath, and we learned Rabbi Yehudah's view that one takes an oath and collects only where there is a partial admission.