Identity theory, a beginner guide

The identity theory gets its name because it identifies - claims an identity between - mental states and certain brain states. I say 'certain' brain state because whilst the identity theory claim that every mental is a brain state, it is not committed to the converse. In fact, it's certainly not the case that every brain state is a mental state. For example, in addition to billions of neurons, the human brain contains a large number of glial cells which play a supportive and protective role. It's unlikely that any mental state is identical with a state of one or more glial cells.

It's fair to say that the identity theory offers convincing explanations of three of the six features, and that it may turn out to be compatible with sophisticated attempts to explain two of the remaining features.

However, one of feature of mental state - consciousness - presents a serious challenge to the identity theory. In this section we will discuss those features of mental states which the identity theory, or a theory compatible with it, can explain.

1. Some mental states are caused by the world. Example: Bloggs's belief that there is a cup of coffee in front of him (mental state) is caused by there being a cup of coffee in front of him (state of the world).

If, as the identity theory claims, mental states are brain states, then the first feature amounts to the claim that some brain states (the ones held by the identity theory to be identical with certain mental states) are caused by states of the world. Research in neuroscience gives us grounds for thinking that this is true. For example, the causal impact of seeing a cup of coffee can be traced deep into the brain. Light from the cup stimulates the light-sensitive cells at the back of the eye (the brain), and information about the pattern of stimulation on the retina is carried into the brain by the optic nerve. (Intriguingly, the pattern of activation on the retina is reproduced many times in the visual centers of the brain.)

2. Some mental state cause actions. Example: Blogg's desire for another coffee (mental state) together with his belief that there is more coffee in the kitchen (mental state), caused him to go into the kitchen (action).

If the identity theory is to explain the second feature of mental states, it must be the case that certain brain states cause actions like going to the kitchen for a coffee. Research in neuroscience makes it overwhelmingly likely that this is the case. We have very good evidence that actions are caused by activity in a part of the brain called the motor cortex.

3. Some mental states cause other mental states. Example: Bloggs's belief that it's Friday (mental state) , together with his belief that Friday is payday (mental state), caused him to believe that it's payday (mental state).

If, as the identity theory insists, mental states are brain states, then the claim that some mental states cause other mental state is supported by the fact that some brain states cause other brain states. However, as we noted in the Introduction, there is something special about the way mental states interact with each other. Notice that my belief that it was Friday, together with my belief that Friday is payday, give me good reason to believe that it's payday. To put this point another way: the causal relations between mental states often respect the rational relations between them. However, it is controversial whether that account of the rationality of thought can be squared with the claim that mental states are brain states.

4. Some mental states are conscious. For the moment we can simply note that some mental states have a subjective 'feel' or 'quality'. Perhaps the easiest way to grasp the idea that some mental states have a subjective feel or quality is to contrast the mental life of a person with normal color vision with that of a person who is colorblind. When they stand together looking at a sunset, the visual experiences of the person with normal color vision are quite different from those of the colorblind person. The former's experiences have a feel or quality which the latter's lack.

5. Some mental states are about things in the world. That is, they represent the world as being a certain way. For example, Bloggs's belief that Mt Everest is 8,848 meters tall is about Mt Everest and represents Mt Everest as being 8,848 meters tall.

6. Some kinds of mental states are systematically correlated with certain kinds of brain states. According to the identity theory, mental states literally are brain states. Consequently, the identity theory smoothly explains the systematic correlation of mental states with brain states.