In this blog post, we will take an in-depth look at the relationship between the mind and the brain through the arguments and counterarguments of mind-body identity theory.

Mind-body identity theory and its counterarguments: The core issue of physicalist philosophy

Mind-body identity theory is a philosophical theory that attempts to explain the relationship between the mind and the body, particularly between psychological states and the physical state of the brain. This theory is based on the claim that psychological states are simply identical to certain physical states of the brain. This explanation is similar to how lightning is an electrical discharge in the atmosphere and temperature is the average kinetic energy of particles, suggesting that what we call psychological states are nothing more than the physical state of the brain.

The identity of mental states and brain states: An empirical fact?

Mind-body identity theory states that there is a correlation between types of mental states and types of brain states, and that this correlation is gradually becoming apparent with advances in neurophysiology. Identitarians argue that this correlation is not a mere similarity, but an identity. In other words, the statement “mental state A is brain state B” is not a simple correspondence, but an identity.
The point to note here is the concept of “identity” as used by monists. This refers to numerical identity, not qualitative identity. For example, “I just bought the same watch you bought yesterday” is a case of qualitative identity, but “That watch is the very watch I lost yesterday” is a case of numerical identity. Identity theory emphasizes the latter concept, that is, that two names actually refer to one object.
It is also important to note that this identity is not conceptually self-evident, but rather an empirically established fact. The statement “a bachelor is an unmarried man” is a conceptual identity, but the statement “water is H₂O” is an empirical identity established through chemical research. Similarly, the claim that “pain is brain state S” is not a conceptual analysis, but an empirical statement based on neurophysiological research.

Leibniz’s law and philosophical objections to identity

Numerical identity is defined by Leibniz’s law. This law states that “if two objects are identical, they must share all properties, and only then are they identical.” Therefore, if “pain” is identical to “brain state S,” then these two states must share all properties.
One counterargument to this is the difference in spatial properties between mental states and brain states. Brain states are physical states and therefore have a spatial location, but it is difficult to specify the location of a mental state such as “pain.” Because of this, some philosophers consider the claim that “pain is brain state S” to be a category mistake, mixing incompatible concepts, like assigning colors to numbers. They argue that just as numbers cannot have the property of color, it is inappropriate for mental states to have physical properties.
However, identitarians respond to this as follows. In the past, it sounded absurd to say that light has a frequency, but now it is a clear fact of physics, and the physical properties of psychological states are gradually becoming an everyday concept. With the advancement of neuroscience and psychology, this “semantic strangeness” is diminishing.

Psychological awareness and intensional fallacy

Another common counterargument is that “I can feel pain, but I am not aware of being in brain state S. Therefore, pain is not brain state S.” This seems convincing at first glance, but it is actually an intensional fallacy, which is a type of philosophical error.
This counterargument merely shows that the two concepts of “pain” and “brain state S” do not provide the same information, but does not prove that they are not actually the same thing. For example, even though we know the concept of “temperature,” our lack of scientific knowledge about the kinetic energy of molecules does not mean that the two concepts refer to different phenomena. Differences in conceptual recognition are not grounds for denying numerical identity.

A strong counterargument to identity theory: multiple realizability

The strongest challenge to identity theory is the argument of multiple realizability raised by Hilary Putnam. This argument questions the premise of identity theory that if pain is a specific brain state, i.e., state S, then beings that do not have that state cannot feel pain.
However, in reality, even mollusks, whose nervous systems are very different from those of humans, appear to feel pain. Furthermore, it is theoretically impossible to rule out the possibility that extraterrestrial life forms with cognitive abilities similar to those of humans but with completely different biological structures, or silicon-based artificial intelligence, could experience pain.
This ultimately leads to the argument that “mental states” should be defined not by the physical basis on which they are realized, but by the functional role they perform.
This position is called psychological functionalism, which is a representative mind-body theory that contrasts with monism.

Monism’s response: Limited identity and scientific analogy

Monists respond to this by acknowledging that the neural basis corresponding to mental states may differ between species, but arguing that this does not completely invalidate monism. They cite the fact that temperature in a gaseous state is the average kinetic energy of molecules. Although temperature manifests itself differently in solid and plasma states, temperature in a gas is still the same as kinetic energy.
Similarly, “pain in humans” may be brain state S, while pain in aliens may be realized in a completely different structure. As a result, the scope of identity has become narrower than expected, but the fundamental claim that mental states are ultimately identical to physical states remains valid.

Conclusion: Identity, functionalism, and the philosophy of mind

Mind-body identity is a theory that has long been the subject of heated debate at the intersection of philosophy and science.
Attempts to reduce mental states to physical states based on empirical identity and numerical identity are scientifically explanatory, but they also reveal their limitations in the face of various ontological possibilities and conceptual complexities. Counterarguments such as Putnam’s multiple realizability argument and functionalism remind us that it is difficult to accept this identification as a universal truth.
Nevertheless, mind-body identity remains a valid philosophical attempt to understand the relationship between the brain and the mind, and it is clear that it is an important theory that has contributed to the development of psychology and neuroscience.
What exactly is a mental state? And how can it exist in the physical world? These questions remain valid, and mind-body identity serves as a sophisticated answer to them.