There is what looks like a contradiction if a person says, ''I know''. (Ketuboth page 22b. Rambam laws of divorce chapter 12, law 19) The Noda BeYehuda says, “I know” has believability for the person himself, but not for others. The first rosh yeshiva of Ponovitch, Rav Shmuel Rozovski disagrees with this. He says I know helps against status quo, but not against a majority. One case is a unmarried woman had sex and gave birth. She and the man say the child is his. Even so, if he is married and then dies with no other children, we say his wife gets halitza, not yibum, because in fact the child might be his [and therefore she needs neither halitza nor yibum]. But perhaps the child is not his and therefore she requires yibum, [since in this case he died without any children.] However, if a woman had sex with a kohen and gave birth to a male child, she can east truma. The Noda BeYehuda gives an answer that Rav Rozovski [of Yeshivat Ponovitch] finds problematic. Instead, says the case where there is doubt if perhaps she had sex with others is where there was a majority of people around in which she might have had sex with. But in terms of “I know” against status quo that is believed [as one Tosphot says in one place and the Rashba also.] I have nothing to add to this except to make a note that in terms of monetary issues, “I know” does have believability to some degree as per Bava Metzia where there is an argument if “I am sure” is believed against “perhaps” is believed even against status quo. (The law is he is not believed.) That is not just for the one who says so. Also, in general we say one witnesses is believed in terms of prohibitions unless the prohibition is established. That believability applies to others, not just to the person himself.-------------------------------------There is what looks like a contradiction if a person says, ''I know''. (כתובות page כ''ב ע''ב. רמב''ם laws of divorce chapter י''ב, law י''ט) The נודע ביהודה says, “I know” has believability for the person himself, but not for others. The first ראש ישיבה of פונובץ' רוזובסקי disagrees with this. He says "I know" helps against status quo, but not against a majority. One case is a unmarried woman had sex and gave birth. She and the man say the child is his. Even so, if he is married and then dies with no other children, we say his wife gets חליצה, not יבום, because in fact the child might be his [and therefore she needs neither חליצה nor יבום]. But perhaps the child is not his and therefore she requires יבום, [since in this case he died without any children.] However, if a woman had sex with a כהן and gave birth to a male child, she can eat תרומה. The נודע ביהודה gives an answer that ר' רוזובסקי finds problematic. Instead, says the case where there is doubt if maybe she had sex with others is where there was a majority of people around in which she might have had sex with. But in terms of “I know” against status quo that is believed [as one תוספות says in one place and the רשב''א also.] I have nothing to add to this except to make a note that in terms of monetary issues, “I know” does have believability to some degree as per בבא מציעא צ''ז ע''ב where there is an argument if “I am sure” is believed against “perhaps” is believed even against חזקה. ברי ושמא ברי עדיף The law is he is not believed.) That is not just for the one who says so. Also, in general we say one witnesses is believed in terms of prohibitions unless the prohibition is established. That believability applies to others, not just to the person himself.