The Mishnah (5:9) records a debate regarding a case where two people testified that an individual became tameh and the person denies it. R' Meir maintains that he is tameh, while the Chachamim argue that the individual is believed and remains tahor. The Tifferet Yisrael explains that it is not just that he is trusted to deal with his own taharot. Rather he even those taharoth that he handled, would be considered tahor for other people as well.
The position of R' Meir is readily understood. The Bartenura explains that if two witnesses have the power to bring about a death penalty, then certainly they should be trusted to render this individual as being tameh. How then do we understand the position of the Chachamim?
The Gemara (Keritut 12a) provides two explanations. The first is that when it comes to matters related to issurim, an individual is completely trusted. The second explanation is that we "interpret his words". In other words, we understand that he means that he was tameh, immersed in a mikveh and is now tahor.
The Mishnah Achrona explains that according to the second explanation, in cases where we cannot interpret his claim in a way that would result in him being tahor, then he would not be believed. For example, if he said, I did not become tameh nor did I immerse in a mikveh. Interestingly the Tifferet Yisarel disagrees. He explains that even if we ask him directly, if he immersed in a mikveh after the incident in question and he replied that he did not, he would still be tahor. We assume that there must be some other reason why he is concealing the tumah and tahara in the face the witnesses.
Another case the Mishnah Achrona raises where he would not be believed, is if the witnesses say that the taharot he engaged in became tameh. Since immersing the taharoth in a mikveh would not make them tahor, then there is no way to favourably interpret his words, and they must be considered tameh.
The Aruch HaShulchan (Shaar Avot HaTumah 151:22) explains that the difference between these two explanations is if the witnesses came and said you just now became tameh. According to the first explanation, the individual is still believed. According to the second explanation however, since there is no way to interpret his words that would make him tahor at that moment, we must say that he is tameh.^1^
The Aruch HaShulchan notes that the Rambam (15:11) rules that even though he is trusted regarding taharot he had already handled, we instruct him to become tahor prior to handling other taharot. This law is presented in the Tosefta. The Aruch HaShulchan argues that this implies that the Rambam maintains first explanation, that the ruling is based on trusting the individual regarding issurim. If the reason was that we "explain his words" -- that he already immersed in a mikveh and became tahor -- there would be no reason to differentiate between past taharot he engaged and those in the future.
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