In the philosophy of mind, the question of the relationship between the physical and mental is at issue. The mind-brain identity theory holds that the relation in question is in fact the identity relation and that the brain (and possibly other parts of the body, like the CNS) literally is the mind.

To a scientific outlook, this can look like a straightforward and obvious statement of fact. Given that minds are real (we hope this much is apparent, even to hard-line materialists) and that if we look with our scientific instruments we find only physical stuff, it is convenient and simple to identify the mind with the physical stuff that we find when we go looking for it scientifically.

But this view does not mesh straightforwardly with our commonsense notions and our normal linguistic practices, hence the identity theory has been the subject of much controversy. Nonetheless, it has been extremely influential, and most current discourse in analytic philosophy on the ontology of the mental has been shaped by its defence, development or opposition.

In the ancient world, Epicurus could be considered a kind of identity theorist, but in its modern formulation, the originators of the theory were Herbert Feigl1, J.J.C. Smart and U.T. Place.

Place took the view that phenomena such as after-images, though real, can't be accounted for in a dispositional (ie. behaviourist) analysis of the mental (which he favoured in the case of concepts like belief and intention.) Finding untenable the traditional dualist view of mental events as distinct from, though correlated with, physical events, he was led to postulate the identity of the former with the latter. His view was that this identification was a scientific theory, in the same way that the identification of lightning with the motion of electrical particles is not a necessary truth, but a contingent one that we establish empirically.

The theory was expanded by D.M. Armstrong in his influential book A Materialist Theory of Mind where it was elevated to a fully fledged ontology. Roughly, Armstrong's view is that all real properties are causally based; he called this the causal theory of properties or CTP for short.

Thus, if we want to say that 'mental properties' (like is seeing a red after-image, for example) are real, then unless we wish to invent some non-physical variety of causality, which Armstrong certainly did not want to do, we have to postulate some corresponding physical property of the brain, in virtue of which our claim to be seeing a red after-image can be thought true.

But, as was pointed out, we need to be clear whether we are identifying mental properties with physical ones or just individual mental events and states with physical ones.

A theory which supports the view that mental properties are real and identical with physical ones is called a type-type identity theory, as it asserts there are types of mental events which are identical with types of physical events.

The more hard-line token-token identity theory, by contrast, is one in which we assert the identity of particular, single, mental states and events with particular single physical ones, but deny that there are necessarily any real physical properties corresponding to the terms we use to classify the mental (terms such as after-image, for example.)

The type-type theories, then, are realist about our descriptions of the mental (or at least they allow realism about these) whereas the token-token theories are not: propositions about the mental can be seen, at best, as useful approximations to the true physical description, or as simply denoting arbitrary disjunctions of physical objects. This has obviously travelled some distance from the identity theory as originally conceived, which sought to preserve the potential truthfulness of statements about the mental, and the token-token theory has been criticised for simply restating epiphenomenalism - the theory that mental events are some strange metaphysical exudence of physical ones - in a hidden guise.

But if, adopting the type-type approach, we say an after-image is identical to a process in the brain, then since the after-image is green we would have to say that the brain-process is green, since identicals share all properties. This objection against the type-type theories is regarded by many as fatal.

Donald Davidson has put forward a theory known as anomalous monism in which, though only token-token identity holds between the mental and the physical, there are nonetheless real physical and mental properties, but with no type-type-like correlation between them.

My own view is that it's the materialist, or physicalist stance that's leading us into trouble here. I am not inclined to say that there are two of me here, a physical one and a mental one, but if we adopt Armstrong's CTP, together with his view (shared, I think, by most identity theorists) that 'causal' means 'investigable by physics', then it seems quite clear that descriptions that are acceptable in physics will not exhaustively describe our experience (see inverse spectrum argument.) So I am led to postulate a 'dual aspect' version of neutral monism, instead.

The most significant current objection to the identity theory, however, is widely thought to be Hilary Putnam's multiple realizability argument, which considers that it would be possible for many different physical states to implement a single mental state, and since the mental states are identical to each other and the physical ones not, the mental states can't be identical with the physical ones. This consideration has probably been influential for those who espouse computationalist theories of mind on the analogy that many different physical arrangements can realise identical computations.


1. see nomological dangler.

The concept of the mind has appeared as a powerful idea that has inspired lively debate among philosophers, psychologists, doctors and even physicists. While this debate has been interesting, none of these groups have reached a consensus on exactly what the mind should be defined as. Thus, for the purposes of this essay, I have chosen to define the mind as the sum of all of the conscious and subconscious mental activity in the forms of internal images, sounds, feelings, smells and tastes that make up an individuals subjective experience of the world. I have chosen this definition for several reasons. The word “mind” itself belongs to a class of nouns termed nominalizations, which are nouns that in fact describe a process rather than a static thing. The definition I have provided avoids this Aristotelianthingness” about the mind and allows us more precise discussion. Second, I believe this definition avoids placing the concept of “mind” into any particular school of thought that may bias the discussion. Given this definition of mind, I propose that it can be neither proven nor disproven that the mind is embedded in the brain. For the purpose of this discussion, "Is the mind embedded in the brain?", I will take that to mean that the existence of mind is dependent upon the brain, and that the mind is localized within the brain.

While it goes without saying that the mind and brain most certainly appear as related, there exists little evidence that the mind and the brain describe the same phenomena. As yet, it can not be proven scientifically that the mind resides locally in the brain of individuals rather than as a non-local phenomenon that merely manifests itself through the nervous system. Eastern mystics call the idea of the non-locality of the mind a “universal consciousness” and some theoretical quantum physicists support this idea as well. Jung’s theories on the collective unconscious and particularly on synchronicity seem to support this view as well. What then of the biological explanation that the mind resides within the brain? The biological explanation makes an assumption that I do not believe: namely that the action of neurons is solely responsible for what we perceive as existence. I do agree that the biological view describes in a very elegant way the mechanisms of the nervous system, but it fails to explain how and why these mechanisms result in our unique experience of the world how we experience it. Where, for example, does consciousness reside in the brain and how does the firing of neurons result in our subjective experience? Further, if the task of experiencing the world is distributed among many different neurons that each has a localized function, how do we sum all of these localized tasks into a continuous and ongoing experience without entertaining the idea of the mind as non-local?

Neuroscience tells us that the brain transduces energy from the outside world into neural impulses. At first, this appears to explain some of the above questions, however upon closer examination we are left with more questions than answers. Neuroscience provides a very complex and detailed picture of the structure of the process of sensing, but fails to address the content. We know, for example, that red light has a particular structure in terms of its wavelength and its effect on certain types of neurons: this is the structure. What we do not know and can not prove is whether my subjective experience of red (qualia) corresponds objectively with someone else’s experience of red: our subjective experience of red is the content, and falls under my definition of mind. Could it be then that the brain provides the structure, while the mind provides the content? If so, then the concept of mind is inherently untestable scientifically, however this may merely be a problem in the definition. The study of the transduction of sound waves into neural impulses reveals another mystery. Certain frequencies of sounds are detected by certain neurons that fire in response to the sound. Other sound frequencies are detected by the rate that the neurons fire. This information is transmitted to the auditory cortex, but how do those impulses get translated into my experience of a symphony? And how might my experience of a symphony differ from somebody else with essentially the same brain structure? How is the information in the neuron decoded into my experience and integrated with the rest of my ongoing experience?

This brings me to my next point. If the mind is not embedded in the brain, then how does the administration of drugs that act on the brain appear to affect the mind? This issue can be resolved in several ways and still remain consistent with my thesis. If the mind is non-local as I discussed earlier, then the apparent effects of drugs on the mind can be explained by the drug’s effect on the mechanism by which the mind manifests itself through the brain. A useful analogy here would be to consider a dam built on a river where the water is the mind and the river valley is the brain. When the dam is built, it changes the structure of the river valley, and interferes with the movement of the water. Another way to approach this issue is again the structure vs. content. Drugs change the physical structure of the operation of the brain and neurotransmitters, if only temporarily. This can be fairly easily predicted in most cases. The content of the drug experience, however, can only be put into generalized categories. When a person ingests LSD we know that they will have some sort of sensory distortions, yet we cannot predict precisely what those sensory distortions will be or what specific things they may hallucinate. Again, this is a difference in structure and content that I find relevant.

One very interesting thing is that the number of neurons containing sense information going forward in the brain is only one-tenth the number of neurons pointing in the other direction. This also ties in with the visual blind spot that we rarely notice. This is just some of the evidence that our experience of the world is a creative process.

I am ending this essay with more questions that I had when I started. The only thing I can be reasonably certain about is that whether the mind is embedded in the brain or not probably depends on your definition of mind. I don’t think this question will be easily resolved until further advances are made in brain research, quantum mechanics (local vs. non-local phenomenon), and a consensus on a scientifically testable concept of mind is reached.

See also: philosphy of mind, Rene Descartes, subjectivism, general semantics

© 2002 Martin Kretzmann

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