This problem would be better named ‘the mind-brain problem’. It is the problem of whether the mind IS the brain, or to be more precise, the problem of whether mental states ARE neural states.
The average modern person holds a ‘knee-jerk’ type-identity theory. In doing so he doesn’t recognise the mind-brain problem as a problem. Surely, he thinks, modern science has shown us that mental state types (say experiences of red) are identical to neural state types (say neural states n)?
There is a major problem for type-identity theory. It is called the ‘multiple realisability problem’. This problem manifests itself in two sub-problems (i) qualia and consciousness, (ii) rationality and content
The Problem of Qualia and Consciousness
When you experience redness there is something it is like to have that experience. Ditto when you have toothache or are in love. No normal human could ever mistake any one of these experiences for the others – what it is like to have them is essential to the having of them. These ‘something it is like’s are called ‘qualia’ (singular: quale).
The traditional example philosophers use to discuss qualia is the example of pains being said to be (identical to) c-fibre firings (CFF).
So a hypothesis of type-identity theory is that pains are CFFs. And our understanding of mental states like pains is that they necessarily have a characteristic qualia.
Given these two premises then, by Leibniz’ law, the indiscernibility of identicals (if A=B, then any property had by A is also a property had by B), CFFs must necessarily have the characteristic qualia of pain.
By the same token CFFs necessarily have a certain physical shape (that by which we recognise them as CFFs). This means, again by the indiscernibility of identicals, that pains must necessarily have the same physical shape.
But do we believe that all CFF necessarily involve qualia? Or that all pains necessarily have the same physical shape? No, we don’t. Nor should we. It is easy to imagine worlds in which pains are realised by physical states other than CFF. This is a world where pain is realised by states other than CFF (in whales for example).
Qualitative states are only one type of mental state. The other type are the so-called propositional attitudes. A belief, for example, is an attitude towards a propositional content. So my belief I am at the computer is an attitude (that of belief) to a propositional content [I am at the computer]. I might have a different attitude to the same content (a desire that I am at the computer). Or I might have the same attitude (belief) to a different content [I am asleep in bed].
Just as qualitative states are identified as states with characteristic qualia, propositional attitudes are identified by their contents (and by the type of attitude they are). Qualitative states essentially have qualia. Propositional attitudes essentially have content.
So let’s say a hypothesis of type-identity theory is that beliefs that P are neural state n. And our understanding of beliefs that P is that they essentially have the content P (if they had any other content, or no content at all, they wouldn’t be beliefs that P).
Given these two premises then, again by the indiscernibility of identicals, neural states n must necessarily have the content P.
And by the same token, instances of neural state n necessarily have a certain physical shape (that by which we recognise them as neural state n). This means, again by the indiscernibility of identicals, that beliefs that P must necessarily have the physical shape of neural state n.
But do we believe that neural states of type n necessarily have content? Or that all beliefs that P necessarily have the same physical shape? Again, no we don’t. And again nor should we. It is easy to imagine worlds in which beliefs that P are realised by physical states other than neural state n. Do we imagine that when we finally meet the little green men they won’t be able to believe P?
Knee-jerk type-identity theory falters immediately on the ‘multiple realisability’ of mental states by physical states. Our intuition tells us that one and the same mental state might be enjoyed by creatures with very different physical bodies. If this is the case then the knee-jerk type-identity theory is false.
We might try to escape this conclusion by relativising pains, and beliefs that P, to humans. So we say pain in humans is CFF, beliefs that P in humans is neural state n. But as science tells us, the brain is plastic enough to counter this claim immediately.
We might again relativise to normal humans. But now we are simply engaging in a forlorn attempt to save the theory. Instead we should abandon the identity claim, recognising that whilst there might undoubtedly be a correlation between pains and CFF, and between beliefs that P and neural state n in normal human beings, this correlation does not provide us with evidence for an identity.
Correlations are pesky beasts. They are, for example, our only empirical evidence for causation. So the recognition that As are correlated with Bs is often unthinkingly taken as evidence for a causal relation between A and B when there is no causal relation between them at all (or the causal relation runs the other way, or when A and B have a common cause).
Correlations are also evidence for theoretical identities. Again this means that the recognition that As are correlated with Bs is often unthinkingly taken as evidence for the claim that As are Bs when in fact As are merely correlated with Bs (perhaps because As cause Bs or for all sorts of other reasons).
I hope this piece serves as a warning against unthinkingly adopting the type-identity theory. I hope also that you’ll recognise that nothing I have said here counts as a refutation of physicalism, the claim that mental states are physical states. There are many types of physicalism other than knee-jerk type-identity theory. Perhaps one of these is the right solution to the traditional mind-body problem.
(See readings at the bottom of this piece)
For those who are gluttons for punishment here is the qualia argument logic-book style:
Premise one: Pain = CFF (hypothesis of type-identity theory)
Premise two: Qualia Q is an essential property of pain/shape S is an essential property of CFF
Premise three: If A=B, then any property of A is also a property of B (The Indiscernibility of Identicals)
Conclusion: Qualia Q is an essential property of CFF/shape S is an essential property of pain
The argument is valid, so the conclusion can be rejected only by rejecting one of the premises. I have suggested that the conclusion should be rejected, and that the only premise we can reject is premise one (the hypothesis of type-identity theory).
As you might imagine philosophers have attacked the argument I am offering in all sorts of ways. How would YOU attack it?