Anon, check out the first piece of meshech chochma in parshas shemos, very fundamental piece of hashkofa. He writes that while we accept neviim, and testimony of witnesses in halacha, it’s a gezeras hakasuv, because even a true navi can go off, like chananiah ben azor, who became a navi sheker and went against yirmiah. Witnesses can lie, and neviim can produce miracles to orove themselves even if they’re lying, so we only believe them because of a gezeras hakasuv.
Not so Moshe rabbeinu and the torah itself; ‘gam becha yaaminu leolam” we believe in nevuas moshe because our ancestors were brought to his level of nevuah, and Hashem told them openly that this is my emissary, listen to his Torah, and of course we heard Hashem giving us the Torah itself. But we don’t believe in it because of krias yam suf or the makos, etc..