Except we have similar evidence of mental decline that does have baseline data--a lot of people participated in The Great British Intelligence Test, before and

Except we have similar evidence of mental decline that does have baseline data--a lot of people participated in The Great British Intelligence Test, before and after scores can be compared. Obviously, we have no way of knowing if the damage is permanent, but the fact that we don't know doesn't give us any reason to think it's temporary.