"But that doesn't make sense because Sally can't be her own sister."

Having said this, how many 'lines' of reasoning does the average human need? It's a weird comparison perhaps but the point is does it really matter if it needs 100 or 100k 'lines', if it could hide that (just as we hide our thoughts or even can't really access the - semi-parallel - things our brain does to come to an answer) eventually and summarise it + give the correct answer, that'd be acceptable?