Token identity theory admits that it is possible that brains may not function in exactly the same way to produce mental states. So, suppose that it is not possible to identify certain types of mental activity – such as seeing a red bus or feeling angry – with a certain type of brain state. Also suppose that it is possible that different people who are in the same mental state have different brain states. Even if all these things are true, it does not mean that identity theory is false.

Consider another possibility. Even though when I have a similar mental experience to you my brain state is very different to yours, it may still be true that my mental experience is identical with my brain state. This is called the type-token theory. In this view, although the brain is still responsible for my mental states, it simply achieves them in a different way. This may lead to a difference between how individuals’ brain states correspond to the same mental states, but also a the same mental state in the same individual may produce different brain states at different times.

The problem with this view is that it is even more open to the benchmark argument than the type identity theory – in fact, there is no benchmark at all, because your brain activity is unique. If I compare what your brain does when you are celebrating a goal and what my brain does – or even what my brain does at different times - there may be very little similarity. This leaves us with an odd picture where similar types of behaviour do not seem to correspond to any particular mental state. Even if it can be argued that each mental state would be slightly different and therefore each brain state will be, it is still a problem for the identity theorists.

The last refuge of identity theory is the token-token theory. This supposes that an individual thought is identical only with the individual brain state which it corresponds to. However, this leaves us with the problem that no brain activity need correspond to any sort of mental state or behaviour whatsoever. This cannot even be tested for, since it may be argued that we can never have the same thought twice – and therefore never the same brain state. Even though this does not disprove the theory, it means that there is nothing to suggest beyond doubt that brain states are mental states.