The document summarizes and evaluates the type-identity thesis, which claims that mental states are identical to physical brain states. It discusses the motivations for the type-identity thesis in response to behaviorism. It examines the arguments of philosophers J.J.C. Smart and D. Lewis in support of the type-identity thesis, including Smart's analogy to lightning and Lewis' argument based on causal roles and transitivity of identity. It also considers criticisms of the type-identity thesis, such as the "dual property" objection and the argument from multiple realizability, and how defenders of the thesis have responded to these criticisms. Overall, the document argues that the type-identity thesis as formulated by D. Lewis withstands

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‘Mental states are identical to physical brain states.’ Citing relevant
texts, explain the motivations of the type-identity thesis and examine
its merits and drawbacks.
Student Number: 1327269
Word Count: 1787
In analytic philosophy we are concerned with how things are. Particularly
interesting is how it is that our brains function. Traditionally, to be involved
in the debate was to choose to be either a Mind-Body dualist or a
materialist, and this was the main contention right up until the early
twentieth century. Here, dualism was all but eliminated as almost all
philosophers of Mind took materialism to be the correct theory, and as such
the debate has moved on; now, the question is not “are our minds material?”
but “how does the mental translate into the physical?” One suggested
answer to this question is the Identity Theory, which I shall be evaluating in
this paper. This is the idea that our mental states are explainable by their
connection to physical brain states, a connection of identity rather than
correlation: identity theorists believe that brain and mind are of one type. I
plan to cover the arguments of J.J.C. Smart and D. Lewis in detail in order
to show that identity theory is the most convincing theory of mind around at
this point in time.
In order to understand the identity theory, it is crucial to be aware of
the one that preceded it, the behavioural theory of mind. This was the idea
that our mental states are nothing but their representation by our actions. A
mental state representing pain can be described as a disposition to cry out,
for example. This was the position that identity theorist were responding to.
One criticism levelled at it is its apparent absurdity: it supplies a much
better way of viewing mentality externally, granting us a much better
understanding of the mental states of others rather than those of ourselves,
when it is commonly held that we have a much better grasp of our own

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mental occurrences than others, and indeed behaviourism fails to show how
we can have direct access to our own mental states.
The identity theory, then, is a statement that, rather than mental states
being fully describable by behaviour of a subject, they can rather be
described by their relation to physical brain states, by way of application of
Leibniz’s Law:
If a=b then any property of a is also a property of b and any property
of b is a property of a.
This does not mean that the mind and the brain are one of the same, but
rather that each and every brain state is identical with some physical state
(Braddon-Mitchell & Jackson 2007, p. 95) There is some contention among
identity theorists about what this actually means. It is key make the
distinction between types and tokens of identity here. Where a token identity
theory would merely state there are certain, standalone, brain states that
are identical other, equally peculiar, mental states, a type identity theory
states that there are certain groups (or types) of brain processes that are
identical to other types of mental states. It is the type theory that I am
concerned with in this paper.
J.J.C. Smart viewed the idea that the relation is one of correlation,
which is a reasonable first assumption to make, as too weak. He believed it
implied that mental states are something ‘over and above’ (Smart 1959, p.
143j) the physical, and indeed you can’t correlate something with anything
that is literally the same, as the identity theory states that brain and mental
states must be. So, Smart argues, mental states must actually be reports of
our brain processes, where the mental state takes on the exact form of the
brain process, in an introspective manner. The key to understanding this
concept is to think of lightning. This phenomena is electrical discharge from
the sky. These are two, seemingly separate, things: the lightning and the
electrical discharge, however lightning is nothing over and above electrical
discharge, and indeed the two are identical. The two concepts are
inseparable in our understanding of the terms. This is also key to showing

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how Smart considers the type identity to show our mental states to be
reports of what is going on in our brains at any one time: in the same way
that lightning is a visual ‘report’ of the airborne discharge of electricity, so
our mental states are a conscious ‘report’ of our brain processes.
There is an immediate problem observable with the type identity
theory: it seems that mental states still must be something ‘over and above’
brain states in order to be something we are immediately conscious of,
where we are not aware of our brain processes. One key objection here is
that the brain process exists in physical space, whereas a mental state does
not. Therefore, they cannot be the same thing. Smart here points out it is
merely a semantic misunderstanding whereby the critic fails to see that two
different words can refer to the same thing with different implications.
Smart gives the example of saying, for instance, that “somebody” is calling
on the telephone, rather than “the doctor.” Both refer to the same person,
the person making the phone call, however they rely on a different
interpretation from the person hearing the reference. (Smart 1959, p.150)
David Lewis was far more dismissive of this dual property criticism.
He calls it ‘metaphysical prejudice that has no claim to be respected.’ (Lewis
1966, p.18) By this claim, he dismisses the criticism that mental states are
necessarily unlocated whilst brain processes must be located somewhere
within our central nervous system. He claims there is no proof that mental
states must be unlocated by nature, and that, as such, the dual property
argument can be completely disregarded.
Lewis’ theory itself is one based on causal roles and the principle of
the transitivity of identity. He argued that the ‘definitive characteristic of any
experience [mental state] is its causal role, and that causal roles themselves
belong to physical states. Therefore, those physical states must necessarily
be the same as mental states. This is an application of the principle of
transitivity of identity such that
if a = b
and b = c

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then a = c
It is not a stretch to see that the logic involved in this principle is valid, and
that, as long as we accept the predicates, Lewis’ argument is sound. The
example he uses for his argument is a cylinder lock. In one of these, setting
the combination causes the alignment of slots which puts it into a state of
unlocking. Here, the alignment of the slots occupies the causal role, and the
state of being unlocked is nothing over and above this alignment (Lewis
1966, p18). Lewis claimed that the causal role of experience is expressible
by finite causes and effects. This allows experiences to be causally linked
which, crucially, brings them into the physical picture as these experiences
can be caused by external factors. Experiences, according to Lewis, could
also be the cause of other experiences, which allows for introspection,
something missing in the behaviourist picture of mind. The beauty of Lewis’
theory is that it gives us a picture of the human mind that is necessarily
connected to the physical world, thus allowing it to cause and be caused,
whilst also allowing for internal reflection.
Many, including Hilary Putnam, have objected to the type identity
theory based on the physical possibilities it entails. They claim the theory is
chauvinistic, in that no theorist had considered that there are other beings
with mental states that haven’t the same kind of brain as humans, such as
fogs, octopuses or robots. The objection is that identity is necessarily a one-
one relation, where one thing (in this case mental states) are identical to
another one thing (in this case physical brain states.) However, dogs don’t
have the same brain as humans do, though they seem to have similar
mental states. Putnam gives the example of a Turing Machine, which can be
programmed to give the same responses as a human (therefore giving its
inputs the same causal roles as the human equivalent.) (Putnam 1973,
p.134) Clearly then, the relation between mental states and physical
processes can be seen as one-many, as the same mental state could be
realised by more than one process. To the sceptic this shows that identity
cannot be true, as in fact mental states are ‘multiply realizable’.

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There are two ways in which to respond to the multiple realisability
criticism. The first is to outright deny its validity, perhaps by claiming that it
is in fact only human like brains that can have processes identical to the
mental states we have. This would mean that anything we call a mental
state in a non-human being would be different to any mental state a person
might have. This, however, is unconvincing, as it seems reasonable to
assume that there are creatures that have similar states to us without
having exactly the same brain structure, perhaps by means of convergent
evolution. The first method would also seem to require that everyone has
exactly the same brain in order for identity to work. So it seems we can
dismiss the first response as unworkable, and move on to the second, the
one endorsed by Lewis (Lewis, 1966). He claimed that in fact, identity holds
between species and different types of brain. In this response, human pain
might be identical to C fibres firing, whereas a squid’s pain would be
identically related to a physical process x or an advanced AI might feel ‘pain’
by some sequence of electrical signals. In each case, the processes aren’t
identical across different species, but are identical to each other within the
individual system. This position takes the realiser of the process to be the
mental state, rather than the realised, as it is the realiser which has the
causal role. This response effectively counters the multiple realisability
objection by accepting and including it into the identity theory. Rather than
denying its plausibility, we just qualify the theory to state that identity
doesn’t try to hold between species, merely within them. There are multiple
ways of realising the same kind of mental states but they are identical
within modes of the same type.
So it can be seen that the type identity theory in the form postulated
by D. Lewis holds up against criticism from those who attacked it for both
the perceived ‘issues’ with dual properties and from its multiple realisability.
It offers a highly considered and well-trod solution to the issue of how
exactly our consciousness works, particularly in its enticing allowance of
mental states as causal roles, something notably left out by its precursor,

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the behavioural theory. On this basis, type identity can be viewed as a
convincing and relevant theory of mind.

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Bibliography
Braddon-Mitchell, D. and Jackson, F.; Philosophy of Mind and Cognition, 2nd
ed. (London: Blackwell, 2007)
Smart, J. 1959; ‘Sensations and Brain Processes’ in Philosophical Review,
Vol. 68 Iss. 2 pp. 141-156
Lewis, D. 1966; ‘An Argument for the Identity Theory’ in Journal of
Philosophy, Vol. 68 Iss. 1 pp. 17-25
Putnam, H. 1973; ‘Philosophy and Our Mental Life’ reprinted in Putnam, H.;
Mind, Language and Reality: Philosophical Papers, vol 2 (Cambridge: CUP
1975) pp. 133-147s