When I read in the Talmud, Shulchan Aruch, etc. that a halacha is halacha leMoshe Misinai (a law given to Moses at Mt. Sinai), I compare it to a "postulate". It seems like a statement or rule that we accept and follow and we can't find a Torah source explaining why we do it.
How do we actually know that this is really in this category and not just a minhag that was adapted by a group of people at some point but we also don't know its source?
And if it is halacha Lemoshe Misinai, how do we know that we are doing it correctly as it was initially relayed to Moshe? Maybe after so many millennia someone relayed it incorrectly and we really don't know?