Really? Everything that might happen could happen. So everything could in some sense be said to be foreseeable. But if so, what is unforeseeable?

It is extremely rare for someone to become psychotic after being hit on the arm. So isn't it really unforeseeable? And yet, shouldn't John be liable? Therefore, can't we say that John is liable for the psychosis even though it is unforeseeable?
Link back to Assignment 6