For a start, it presumes that it is theoretically possible to observe what is going on in your brain and for that to mean the same thing as what you are actually experiencing. Leaving aside the technological possibility of such a feat for the moment, could such an account of my brain states be identical to my thoughts?

If we imagine two people seeing a red bus, we should really be able to say that the same thing is happening in both brains. However, perhaps seeing a bus means different things for each person – perhaps one thinks, “Oh no! I’ve missed it!” whilst the other thinks, “Good, I’ve missed it. I didn’t really wan’t to go anyway.” As you can see, having the thought, “I see a red bus” can mean very different things for people, so how can the same brain state (seeing the red bus) mean the same thing for both people?

The Identity Theorist would argue that obviously there is no such thing as simply “seeing a red bus” and that of course each experience can have complex personal relevance. However, the total reaction of the individual – seeing the bus and feeling relief/disappointment – would register as a brain state and would therefore differ between individuals.