[Phil153]

Thanks for the answers, it's clearer now. But the theory is still horribly confused and not at all stated clearly. To summarize:

1. "We" are identical to our biology (nihilism).
2. Persistence of an organism depends on whether its biological functions are sustained.
3. Therefore, our mental life has nothing to do with our persistence

The argument of Animalism basically begs the question by denying the existence of separate thing called "mental life" or identity, reducing everything to biology. It's also ill defined by not defining organism properly. Is it the majority of cells (how does that work for a fat person undergoing liposuction?) Is it certain cells?

Basically, animalism as a theory does not exist because it doesn't define itself.

So the only thing the "brain transplant" scenario shows is "organism" in animalism is poorly defined. It certainly doesn't refute it. You could define a version of animalism in which it's claimed that we are just the biology of our brains, and the rest of our body is just a support function. This would retain the core elements of the theory and get around the brain transplant refutation.