Nazir 63A

מתני׳ נזיר שגילח, ונודע לו שהוא טמא, אם טומאה ידועה ⁠— סותר, ואם טומאת תהום ⁠— אינו סותר. אם עד שלא גילח, בין כך ובין כך ⁠— סותר.

Mishnah: With regard to a nazirite who shaved for the conclusion of his naziriteship, and it later became known to him that during his naziriteship he was ritually impure from a corpse, if it was a known impurity, i.e., people were aware of the impurity when he became impure, he negates his entire naziriteship. And if it was ritual impurity imparted by a grave in the depths, one that was unknown at the time, he does not negate his naziriteship. If he discovered he was impure before he shaved, he negates his naziriteship in either case.

כיצד? ירד לטבול במערה ונמצא מת צף על פי המערה ⁠— טמא, נמצא משוקע בקרקע המערה,

The Mishnah asks: How does one differentiate between a known and an unknown impurity? If a nazirite descended to immerse in a cave, and a corpse was found floating at the mouth of the cave, he is impure, as an openly visible corpse is a known impurity. What, then, is an impurity of the depths? This is referring to a case where the corpse was found sunk into the ground of the cave in such a manner that it was unknown.

ירד להקר ⁠— טהור. ליטהר מטומאת מת ⁠— טמא. שחזקת טמא ⁠— טמא, וחזקת טהור ⁠— טהור, שרגלים לדבר.

However, even here the circumstances of the case must be taken into account. If one descended not to immerse himself in the water, as he was ritually pure, but to cool himself, he remains pure. If he was impure and entered the water to purify himself from the impurity from a corpse, he is impure. The reason is that something that has the presumptive status of impurity remains impure, and something that has the presumptive status of purity is pure, as there is a basis for the matter. It is reasonable that items or people retain their presumptive status.