Man found missing 90% of his brain defies scientific understanding - most of his skull is filled with water:
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(07)61127-1/fulltext
He's not 100% normal, you just haven't experienced the things he cannot process properly or at all, which is likely a lot of higher reasoning... They just don't affect his day to day, which gives us clues that day to day functioning is very low level and likely mostly autonomous.