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1) In regard to the question of why even a Talmud Chacham must state k'nega, like a mark, rather than just call it in a straightforward manner a nega -- after all he knows the criteria -- there are many different answer with significant moral lessons. For example, there is the argument that since the scholar still must wait for the kohain's ruling, this teaches a certain level of humility -- don't run to state the conclusion until you have to. You can always be missing something. On this overall question, see, further, Rabbi Moshe Hochman, Morasha, Parshat Metzorah, K'nega Nireh Li.