Def 1: Type-identity theory: brain states and mental states are the same thing. The mental state "I love Metallica" is nothing more than the neural configuration of a Metallica fan.
Def 2: Multiple Realizability: that the same mental state can be realized in more than one type of system, a human brain, an animal brain or an AI.
I was listening to a lecture on the mind-body problem, and the speaker stated that the concept of Multiple Realizability definitively refuted the type-identity theory of the mind. That once the Multiple Realizability argument was made, people started looking for other possible physicalist answers to the mind body problem (e.g. functionalism, the computational theory of mind, etc...) and gave up on type-identity.
What I can't wrap my head around is, I don't see why Multiple Realizability refutes type-identity.
1 - There is no reason why Multiple Realizability should hold: for all we know, human brains are the only systems capable of realizing human mental states.
2 - The fact that animal brains, AI or alien minds made of some other form of matter can have thoughts and sensations analogous to human thoughts and sensations doesn't mean that human mental states form an ontologicaly distinct category from human brain states. For example, the fact that there are blonde wigs made of nylon and lions with yellow fur, doesn't mean that "blondness" is a different category from "having hair with a certain type of pigmentation".
So my questions are: What arguments are given for the necessity of Mutiple Realizability? And, even if it does hold, why does Multiple Realizability imply that mental states are different from brain states?