Kripke begins his argument against the mind-brain identity thesis by explaining what a rigid designator is.
First, he defines necessary and contingent truths. Necessary truths are true in all possible worlds, while contingent truths simply happen to be true in this world but could be false in another. Then, he explains that a rigid designator is something that refers to the same object in all possible worlds. Finally, Kripke concludes that if two rigid designators are referring to the same object or concept, then the statement must be a necessary truth.
Even if the statement is discovered posteriori, or empirically, it is still necessarily true. This is different than previous understandings of necessary truths, which philosophers perceived needed to be a priori in order to be necessarily true. The reason that states of consciousness, such as pain, are special in Kripke’s argument is because they are rigidly designated by their experiences. For example, since we cannot imagine pain that does not feel like pain, pain and the feeling of pain are both rigid designators that refer to the same concept. Thus, it is necessarily true that pain is the feeling of pain.
This is unique because it is different than many other things we can experience in the world; for example, we can conceive of heat that is not the motion of molecules, which we did before we discovered that they are the same thing. However, there is no way that we can think of pain that is not the feeling of pain, because the feeling of pain is its essential property. Smart, on the other hand, does not think of states of consciousness as anything special. Smart argues that the mind and brain are one and the same in his mind-brain identity thesis. He says that sensations are simply brain processes, and that pain, for example, is merely C-fibers firing in the brain. He believes that brain processes are not merely correlated with sensations, but that they are sensations. This is a type of physicalism. Physicalists believe that the brain and therefore the mind can be thoroughly explored through scientific means, since studying the brain means studying the mind.
Personally, I agree with Kripke’s view and disagree with Smart’s view. I think that it is impossible for pain to exist without a person there to feel it. For example, if what Smart says is true, then the feeling of pain should exist even if C-fibers fire somewhere random outside of a brain and a person’s body. However, since there is no one there to experience the pain, how can it be said that the pain was really caused? I believe that one can only say that there was C-fiber firing, but no pain. This contradicts Smart’s argument that C-fiber firing and pain are the same thing.
The special thing about states of consciousness is that they only exist in relation to someone able to experience them. States of consciousness cannot exist in a void, because their very definition is based on the feeling of the person experiencing them.