I might just add one further observation here. At the risk of sounding pedantic, 'happiness' and 'sadness' are not adjectives (as you suggest). They're nouns. The corresponding adjectives here are the words 'happy' and 'sad'. Now, I would agree with you that there seems to be something deeply peculiar about a sentence like "this is happy brain tissue". Admittedly, and as Sean Greenberg indicates, philosophers don't tend to seek to reduce mental states simply to brain tissue but rather to states of that tissue. But still, that doesn't help: the sentence "this is a happy brain state" or "this brain state is happy" doesn't sound much less jarring. However, I think the reason why these sentences sound so harsh is not because we're here talking about a neurological state as opposed to a mental one. It would strike me as equally peculiar to say "this mental state is happy". That's because I disagree with your suggestion that we apply these adjectives to mental states at all. We do apply the nouns to them: the word 'happiness' is just the name of a certain mental state. But, when it comes to the adjectives, these are terms that we apply to people, not to states of those people. The proper thing to say is surely something like "this person is happy" or "this is a happy person". And the occasion for saying such a thing is when the person is in the state that we call 'happiness'. But this doesn't seem to depend in any way on the ontological status of that state. Regardless of whether happiness should turn out to be just a mental state, or alternatively a state that is in some way both mental and physical, we could still say all of the same things about the person: "this person is happy", "this person possesses happiness", etc.
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Many philosophers think that mental states can be reduced to physical states. It seems to me however that properties such as sadness and happiness are adjectives that apply to a person's mental states. It doesn't make any sense to say "this is happy brain tissue" does it?