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The theory states that when we experience something - e.g. pain - this is exactly reflected by a corresponding neurological state in the brain (such as the interaction of certain neurons, axons, etc.). From this point of view, your mind is your brain - they are identical. Mental events are in fact physical events. Modern technology allows us to map brain activity to specific areas of the brain. MRI  has allowed scientists to study the structure and activity of the brain in detail. It is able to track blood flow. This allows us to see which areas of the brain are active when certain prescribed activities are performed.

Type identity

Type physicalism asserts that mental events can be grouped into types, and can then be correlated with types of physical events in the brain. For example, Everytime anyone is happy, there is the same corresponding brain state. It is sometimes called the “identity thesis” because it asserted an identity between mental states and brain states.

Critiques

Is it likely that the brain structures of all mammals, reptiles, birds, amphibians and molluscs realize pain, or other mental states, in exactly the same way? Do they even have the same brain structures? Clearly not. How is it possible then that they can share the same mental states and properties? The answer had to be that these mental kinds were realized by different physical states in different species.

Can we really say that all my happy moods have something in common? If I write down the defining characteristics of all my different moods, won’t I find that some very different moods have a lot in common (fear and excitement, for instance)? So, doesn't this suggest that – even if brain states are mental states – all happy states might correspond to a range of very different brain states? And this idea was responsible for a refinement of the theory known as Token Identity Theory.

Token identity

Token identity physicalism, argues that mental events are unlikely to have "steady" or categorical biological correlates. These positions make use of the philosophical type–token distinction (e.g., having the same "type" (abstract general entities ex. car) need not mean same "token" (particular objects ex. BMW or a certain vehicle). A token of a type is a particular concrete exemplification of that abstract general type. We can see how the identity theorists were motivated to move from a type-type identity theory to a token-token identity theory. The token-token identity theorists did not require, for example, that all token pains had to exemplify exactly the same type of brain state. They might be tokens of different types of brain states even though they were all tokens of the same mental type, pain.

Critiques

The first technical objection was that the theory seemed to violate a principle of logic called “Leibnitz’s Law.” The law says that if any two things are identical, then they must have all their properties in common. So if you could show that mental states had properties that could not be attributed to brain states, and brain states had properties that could not be attributed to mental states, it looks like you would refute the identity theory. For example, for conscious states that have a location, such as pain, the pain may be in my toe, but the brain state that corresponds to that pain is not in my toe, but in my brain. So the properties of the brain state are not the same as the properties of the mental state. Therefore, physicalism is false.

Putnam

In several papers published by Hilary Putnam in the late 1960s, he argued that, contrary to the famous claim of type-identity theory, it was not true that "pain is identical to C-fibre firing." It is possible that pain corresponds to, or is at least correlated with, completely different physical states of the nervous system in different organisms and yet they all experience the same mental state of "being in pain." Putnam cited numerous examples from all over the animal kingdom to illustrate his thesis. Is it likely that the brain structures of all mammals, reptiles, birds, amphibians and molluscs realize pain, or other mental states, in exactly the same way? Do they even have the same brain structures? Clearly not, if we are to believe the evidence furnished by comparative neuroanatomy and neurophysiology. How is it possible then that they can share the same mental states and properties? The answer had to be that these mental kinds were realized by different physical states in different species.

Thus, if we can find even one psychological predicate which can clearly be applied to both a mammal and an octopus (say, “ hungry ” ), but whose physical – chemical “ correlate ” is different in the two cases, the brain state theory has collapsed. It seems to me overwhelmingly probable that we can do this. (Putnam, 436) 

P1. If type - physicalism is true, then every mental property can be realized in exactly one physical way.
P2. It is empirically highly plausible that mental properties are capable of multiple realizations.
C1. It is (empirically) highly plausible that the view of type - physicalism is false ( modus tollens , P1, P2).

In addition to undermining type - physicalism, Putnam ’s argument paved the way for the functionalist view of the mind.

Qualia

Given the scientific identification of heat with the motion of molecules, there is no further explanation that needs to be given:

"our knowledge of chemistry and physics makes intelligible how it is that something like the motion of molecules could play the causal role we associate with heat…. Once we understand how this causal role is carried out there is nothing more we need to understand." (Levine 1983) 

In contrast, when we are told that pain is to be identified with some neural or functional state, while we have learned quite a bit, there is still something left unexplained. Suppose, for example, that we precisely identify the neural mechanism that accounts for pain—C-fiber firing, let's say. Still, a further question would remain: Why does our experience of pain feel the way that it does? Why does C-fiber firing feel like this, rather than like that, or rather than nothing at all? Identifying pain with C-fiber firing fails to provide us with a complete explanation along the lines of the identification of heat with the motion of molecules. Erwin Schrödinger (1887–1961), the famous physicist, had this counter-materialist take:

The sensation of color cannot be accounted for by the physicist's objective picture of light-waves. Could the physiologist account for it, if he had fuller knowledge than he has of the processes in the retina and the nervous processes set up by them in the optical nerve bundles and in the brain? I do not think so.

This qualitative feel is called “qualia,” of which the singular is quale. Qaulia are the contents of your subjectiive experience, how the world looks and feels to you. Examples for it include how pain feels, how red looks, how a rose smells. There is a qualitative feel to drinking juice, which is  different from the qualitative feel of listening to Beethoven’s Ninth Symphony. Qaulia cannot be communicated, or apprehended by any other means than direct experience. For example colors, could you describe how a color looks to a blind person? you can only explain it to someone who already knows it. All materialist theories like functionalism deny their existence, so they are false. A full look on their types may include:

1. Perceptual experiences, for example,experiences of the sort involved in seeing green, hearing loud trumpets smelling the sea air, running one's fingers over sandpaper. 
2. Bodily sensations, for example, feeling a twinge of pain, feeling an itch, feeling hungry, having a stomach ache, feeling hot, feeling dizzy. Think here also of experiences such as those present during orgasm or while running flat-out. 
3. Felt reactions or passions , for example, feeling delight, lust, fear, love, feeling grief, jealousy, regret. 
4. Felt moods, for example, feeling happy, depressed, calm, bored, tense, miserable.

Qualia can’t be analysed in terms of functional role; so functionalism can’t explain it, it would leave out the subjective, qualitative, first-person, experiential phenomena. Consciousness involves a ‘point of view’, and there is something it is like, for a conscious creature, to be that creatures. Feeling pain causes you to cry out or withdraw your hand from the fire. But the feeling of the pain isn’t just these causal relations.

Mary's room

Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘ red ’ , ‘ blue ’ , and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence ‘ The sky is blue ’ .What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? It seems just obvious that she will earn something about the world and our visual experience of it. But then it is inescapable that her previous knowledge was incomplete. But she had all physical knowledge. Ergo there is more to have than that, and physicalism is false. (Jackson “ Epiphenomenal Qualia, ” 130)

The point of the argument is that there exist real phenomena that are necessarily left out of the scope of their knowledge, as long as their knowledge is only of objective, third-person, physical facts. The real phenomena are color experiences and the bat’s feelings, respectively; and these are subjective, first-person, conscious phenomena. The problem in Mary’s case is not just that she lacks information about some other phenomenon; rather, there is a certain type of experience that she has not yet had. And that experience, a first-person subjective phenomenon, cannot be identical with the third-person, objective neuronal and functional correlates.

Zombie Argument 

Many philosophers, including Chalmers in his book "The Conscious Mind", have recently claimed that we can coherently imagine the existence of zombies. This claim is taken to imply the possibility of zombies, a claim that in turn is taken to imply the falsity of physicalism. The zombies are by definition exactly like us physically. But if two creatures alike physically can differ with respect to consciousness, then it seems to show that consciousness is something over and above the physical. This argument says it is conceivable that my body could exist and be exactly as it is, but without my mind, therefore my mind is not identical with my body, or any part of, or any functioning of my body. It is sometimes suggested that God could have created a zombie world, if he had so chosen. From here, it is inferred that consciousness must be nonphysical. If there is a metaphysically possible universe that is physically identical to ours but that lacks consciousness, then consciousness must be a further, nonphysical component of our universe. If God could have created a zombie world, then (as Kripke puts it) after creating the physical processes in our world, he had to do more work to ensure that it contained consciousness.

Zombie-like, non-conscious creatures that do not possess “qualia”. Such creatures, whilst fitting the Functionalist criteria for possessing a mind, could not – non-functionalists argue – be said to be human in the full sense (thereby implying that the Functionalist view is inadequate). For example, when Zack and Zombie Zack each take a bite of chocolate cake, they each have the same reaction—they smile, exclaim how good it is, lick their lips, and reach for another forkful. But whereas Zack, a phenomenally conscious being, is having a distinctive (and delightful) qualitative experience while tasting the chocolate cake, Zombie Zack is experiencing nothing at all. This suggests that Zack's consciousness is a further fact about him, over and above all the physical facts about him (since all those physical facts are true of Zombie Zack as well). Consciousness, that is, must be nonphysical.