A nazir who shaved, and it [then] became known to him that he was tamei: if [it was due to] a known tumah —- he forfeits; if [it was due to] a tumah of the depths, he does not forfeit. If he had not yet shaved [when it became known to him, then] regardless of which [tumah] it was, he forfeits. How so? [If] he descended into a cave to immerse himself and a corpse was found floating at the mouth of the cave, he is tamei. [If] it was found sunken in the floor of the cave —- if he had gone down to cool himself, he is tahor; to purify himself from tumah of the dead, he is tamei; because someone tamei is presumed to remain tamei, while someone tahor is presumed to remain tahor, for the matter is indicated.