Types of Identity Theory

Type identity occurs when two or more things belong to the same class. Token identity is when two or more things are exactly the same. For instance the sentence A rose is a rose is a rose contains 8 examples of types of words but only 3 tokens; 'a', 'rose' and 'is'.

Type-type mind brain identity assume that 

• a brain state called 'happiness' exists
• happiness as experienced by me will share type identity with happiness as experienced by you. 
• The same brain state will occur whenever happiness occurs. 

The two main problems with this view are

1. do all my moments of happiness have something in common? (For instance, think about the happiness you get from watching a sunset and from riding a roller-coaster). Even if brain states are mental states a simple one to one correspondence might not be possible.
2. can we be sure that the same mental states produce the same brain states in other people? Magnetic resonance scanners allow us to see which part of the brain is active; if I think about the colour red a certain part of my brain will light up. If the same part lights up in someone else does this mean that they are also thinking about red? My idea of red might not be exactly the same as theirs - would this invalidate the scan comparisons? This is known as the benchmark problem.

In the examples above happiness can be constructed in different ways using different tokens of the same type.

Type-token mind brain identity theory admits that the same mental state might have different causes; brain scans taken when I feel happiness might only have some things in common but they will look completely different from scans taken when I am bored or disgusted. This view suffers from the same problem as type-type identity theory in that benchmarking is almost impossible, but it does admit the possibility of multiple realisability. I can experience happiness; so can you; so can that dog; and so can that robot if it has a sufficiently advanced brain. 

In this example happiness is produced in different forms but still by the type of thing you might find in your pocket


Some identity theorists insist that the same mental state must have the same brain state in all cases. This is known as token token identity theory but it is impossible to prove even theoretically as no two instances of happiness are ever the same even in the same individual.

 

No two tokens can ever really be identical even if they have the same value.