All of these early proponents of identity theory advanced what is now known as a type identity theory. This theses stated that minds are equivalent to brains, for every type of mental event there must be a corresponding type of brain event. Just associating mental states with brain states simply isn't enough; they are in fact type identical. Every single mental state is equivalent to a physical event, namely a particular brain event (Identity theorists might extend the brain to include the Central Nervous System). Part of the reason why this reductionism was so attractive is that great progress had been made in other physical sciences. An example of a successful reduction would be reducing the complicated concept of heat to mean molecular motion. If the same could be done for mental states it would justify the feeling among the scientific community that mental states are in fact brain states.
An example of such a reduction often proposed by the original type identity theorists is that of C-fibers firing being type identical to pain. Though we now know that this is bad neuro biology and in fact different types of pain involve the firing of different fibers. The statement that the firing of C-fibers is type identical with pain, is a form of reductive physicalism and claims that the realm of the mental is nothing more than the physical and that the mental can be totally reduced to the physical. You can see that in a way this is more radical than Logical Behaviourism, which can be put forward simply as a claim about the meaning of mental terms.
Those who adopted the identity thesis were unable to put forward any empirical arguments for it. This was due to the fact that the most that can be empirically established is a correlation between mental states and brain states. It is perfectly possible that empirical science could refute the correlation as it did with C-fibers being identical with pain. Rather the identity theorist offers philosophical arguments in favor of identity theory. Here we are going to run through a number of arguments offered up as a reason to adopt an identity theory of mind.
- The Argument from Explanation: Favored by J.J.C Smart, he contends that we should adopt identity theory by appealing to Ockham's Razor. It possible for us to explain everything in terms of the physical without having to appeal to special mental entities which lie outside the physical realm.
- The Argument from Causation: Noting that certain physical events are caused by mental events and appealing to the causal closure of physics, we are supposed to conclude that all mental events are equivalent with physical events.
But there are a number of problems facing the type identity theories of mind. So have contended that type identity theories are chauvinistic. This is due to the fact that they deny the possibility of multiple realizability. For example say we accept that the firing of C-fibers is equivalent to pain, we seem to be denying the possibility of animals such as dogs having pain. This is due to the fact that other animals do not have the same neuro biology as us and pain may be realized in different ways in different species. This means the firing of c-fibers is not necessary for pain. It seems that this problem can be avoided by introducing an element of species relativity to the theory, so instead of saying that pain is equivalent to C-fibers we say that pain in human is equivalent to C-fibers firing and pain in dogs is equivalent to the firing of D1 fibers.
But a species relative type identity theory may still be to restrictive. For example it is possible that pain in me could be realized in a different way from how it is realized in you. There seems to be plenty of empirical evidence showing that different parts of the brain can take over functions typically associated with other parts of the brain in patients with brain damage. One example would be patients who experience the phenomena known as blindsight. Another objection that can made against type identity theory, namely that it assumes the completeness of physics (the belief that physics will ultimately give us an explanation of all phenomena). Clearly our current physics doesn't give us a complete explanation of phenomena. Claiming that physics means the complete theory of everything given to us at the end day, is merely trivial to claim that everything is physical.
In response to the many of the objections made against type identity theories, many identity theorists adopted a weaker form of the doctrine. Token identity theories are weaker in the sense that a token identity theorist doesn't hold that a given mental state is identical with a particular brain state, but rather that mental states are identical with some kind of brain state. In its very weakest formulation a token identity theorist holds that every time that an individual is in some mental state the subject must also be in a some kind of physical state. But generally most token identity theorists want to make a stronger claim namely that every mental event is dependent on some kind of physical event. This an asymmetric relationship as the mental depends on the physical, but the physical is not dependent on the mental. This is normally described by saying that the mental supervenes on the physical. Though there are many problems relating to how we are meant to spell this out (See Horgan).
A number of problems remain for the token identity theorist, including how fine grained should any of the proposed token identities be, how the theory will impact on generalizations which folk psychology relies (for example it could become apparent that there every pain is a unique one off). But the problem that really led to the demise of identity theory is Kripke's critique of materialism, which I will not go into here as to properly cover the subject matter would require a dedicated post.