> If two thing cannot possibly be observed to be different then they are the same.
I think that suffers the same flaw as logical positivism: if my axioms can't find a difference, there isn't one, no way my assumptions are wrong. (Namely, my axiom is that external observations capture the entirety of reality, there is nothing subjective.)
If two people laugh at a joke, one faking and one actually finding it funny, what is the externally observable difference? Assume the faker has been trained in all manner of knowledge about what would make the joke funny, they just don't find it so.