Avodah Zarah 42A

של מציק אחד ברימון, שהטילה נפל לבור, ובא כהן והציץ לידע אם זכר אם נקבה, ובא מעשה לפני חכמים וטיהרוהו, מפני שחולדה וברדלס מצוין שם.

of one violent person [metzik] in the city of Rimon, who cast a non-viable newborn into a pit, and a priest came and looked into the pit to ascertain whether the baby was male or whether it was female, as the length of time of a woman’s ritual impurity after childbirth, even if she gave birth to a non-viable newborn, depends on whether the child was male or female (see Leviticus, chapter 12). And the incident came before the rabbis to rule whether or not the priest had contracted ritual impurity while standing over the corpse, and they deemed him ritually pure. The basis for this ruling was due to the fact that as martens and polecats [bardelas] are common there, it is likely that the body was dragged away before the priest arrived at the pit.

והא הכא, דודאי הטילה נפל, ספק גררוהו, ספק לא גררוהו, וקאתי ספק ומוציא מידי ודאי!

The Talmud concludes its objection: And here, in this case, where it is certain that the woman cast the non-viable newborn into the pit, and it is uncertain whether an animal dragged it away and it is uncertain whether no animal dragged it away, the rabbis nevertheless ruled that an uncertainty comes and overrides a certainty.

לא תימא הטילה נפל לבור, אלא אימא: הטילה כמין נפל לבור.

The Talmud rejects this interpretation of the baraita: Do not say that the woman certainly cast a non-viable newborn into a pit; rather, say that she cast an object similar to a non-viable newborn into a pit. Perhaps it was not the body of an infant; it might have merely been congealed blood, which does not impart impurity. Therefore, it is a conflict between uncertainty and uncertainty; it is unclear whether the item that was cast into the pit could have rendered the priest ritually impure, and even if it could have, it might have already been dragged away.