© Michael Lacewing
Mind–brain type identity theory
Physicalism claims that everything that exists is dependent on something physical
in order to exist. ‘The mind’ is not a separate substance, a ‘thing’. It is more
accurate to talk of mental properties – mental events, states and processes. We
can then say that these properties are possessed not by a mind, but by a person
or a brain, which are physical objects. While physicalists agree on this, they
differ on what mental properties are and how they relate to physical properties.
TYPE IDENTITY THEORY
A swan is a bird and (usually) white – but what makes it a bird (a biological
property) and what makes it white (a colour property) are different properties,
though both are physical properties. So there are lots of different kinds of
physical properties. We can ask, are mental properties a kind of physical
property? For example, can thinking a thought, holding a belief, or feeling an itch
be neurophysiological properties?
‘Type identity theory’ claims that mental properties just are physical properties.
If we say that these physical properties are properties of a brain, then the theory
is mind–brain type identity theory. So this theory claims that thinking a thought or
feeling an itch is exactly the same thing as certain neurons firing and having a
belief is the same thing as certain neural connections existing. Any particular type
of mental state is a particular type of brain state.
Mind–brain type identity theory was developed in the 1960s as neuroscience
gathered pace. The evidence is that mental events and states are very closely
dependent on the brain, so many people now think that ‘the mind’ is just ‘the
brain’, and everything mental is actually neurophysiological.
However, this claim needs to be distinguished from the claim that mental states
are correlated with brain states. For example, having a heart is correlated with
having kidneys – every animal that has a heart has kidneys and vice versa. But
hearts and kidneys are not the same thing! Or again, having a size and having a
shape are correlated – everything that has a size has a shape and vice versa. But
size and shape are distinct properties. So simply pointing out that everything that
has a particular brain state also has a particular mental state doesn’t show that
mental states and brain states are the same thing. Correlation is not identity.
Neuroscientific evidence can establish correlations between mental and physical
properties, but this does not establish the type identity theory. The mind–brain
type identity theory doesn’t claim that mental properties are correlated with
certain physical properties. It says they are identical with them. But to
demonstrate identity, we need to think philosophically, not scientifically.
The theory is called ‘type’ identity, because it claims that mental ‘types’ of thing
(mental properties, states and events) are physical ‘types’ of thing – physical
properties, states and events. Mental ‘things’ turn out to be the same type of
‘thing’ as physical ‘things’, i.e. mental properties are actually physical properties
of the brain, mental states are brain states. They may not seem the same, but
that’s because we have different ways of knowing about these properties – through
experience and through neuroscience. Many things turn out to be something they
don’t seem to be, e.g. solid objects are mostly empty space, water is just
hydrogen and oxygen (who’d have guessed?).
Type identity theory is a form of ‘reduction’. An ‘ontological reduction’ involves
the claim that the things in one domain (e.g. mental things) are identical with
some of the things in another domain. For example, we can argue that heat is
just mean molecular kinetic energy. They are the same thing. Or again, although
they seem different, electricity and magnetism are the same force,
electromagnetism. Every mental property, type identity theory argues, is a
certain physical property. There is nothing more to mental properties than being
a certain kind of physical property.
SMART, ‘SENSATIONS AND BRAIN PROCESSES’, pp. 141–8
J. J. C. Smart defends mind–brain type identity theory for sensations, but the
theory can be generalized to other mental states and occurrences. The
motivation for the theory, Smart says, is simply Ockham’s razor. If there are no
overwhelming arguments in favour of dualism, then we should reject the idea of
distinct non- physical substances or properties. Science indicates that the
physical properties of the brain are a good candidate for what mental properties
are.
Smart’s claim is that ‘[s]ensations are nothing over and above brain processes’
(145) – not correlated, but identical. To understand the theory, it is important to
get clear on the identity claim. It is not a claim about language or concepts, but
about reality. So the claim is not, for example, that the concept ‘pain’ means ‘the
firing of nociceptors’ (a type of neuron involved in pain). The identity theorist is
not offering definitions of the terms that we use. The identity claim is, therefore,
not meant to be analytically true. ‘Pain is the firing of nociceptors’ (if it is true) is
not true in the same way that ‘bachelors are unmarried men’ is true. The concepts
‘pain’ and ‘the firing of nociceptors’ remain distinct. The claim is that both
concepts refer to the same thing in the world. The firing of nociceptors is what
pain is. Two concepts, one property.
Understanding the nature of the identity claim helps deal with objections. In
particular, we might object that we can talk about sensations without knowing
anything about brains. So they can’t be the same thing. But, replies Smart, we
can talk about lightning without knowing anything about electrical discharge –
because the concepts are distinct. That doesn’t show that lightning isn’t
electrical discharge. The same is true of ‘sensation’ and ‘brain process’.
IDENTITY THEORY AND MENTAL CAUSATION
Both substance dualism and logical behaviourism face difficulties explaining
mental causation. On the one hand, if minds and bodies are distinct, it is hard to
see how events in one could cause events in the other. On the other hand, we do
not want to deny, as Ryle’s logical behaviourism does, that there is any mental
causation.
Type identity theory claims to solve the problem. All mental properties are
identical with brain properties. Mental occurrences are identical with neurons
firing. Mental states that involve behavioural dispositions are neurological
connections. And so all mental causation just becomes a form of physical
causation. For my desire for food to cause my searching for food is just for certain
physical properties of my brain to cause that behaviour. To say my decision to
watch TV is the cause of my picking up the remote control is just to say that some
particular event in my brain with certain physical properties is the cause of my
picking up the remote control. Mental states and processes cause actions because
they are physical states and processes.