The mind-brain Identity thesis: Problems and solutions

What is the mind, which gives humans this rich conscious experience, and how is it related to physical bodies?  This ‘true nature’ of the ‘mind’ has intrigued man for centuries, and even now, with a much greater knowledge of the world, the answer still eludes those searching.  Many attempts have been made to answer these existential questions, and although none have been completely successful, there are theories regarded as more plausible. One such theory is the ‘mind-body identity thesis’, a fully materialist theory which claims that all mental states are physical states.  Like all theories, it has come against considerable opposition, with objections ranging from alleged contradictions of Liebniz’s law, through to problems of representation and identity.  The identity thesis (arguably) has a sound response to these objections, and so offers a clear, rational, and logical theory to answer the problem of how our mind and body are connected.  However, there are still some weaknesses that have not yet been recognised as successfully overcome.  In this, at least, the identity thesis is not alone; there are still problems with all other alternatives.  The problems faced by the identity thesis will be explored, along with its best responses.  Firstly it is necessary to explain the identity thesis, and place it in the wider context of theories of mind.

The identity thesis is a fully materialist theory claiming the contingent[1], a posteriori[2] proposition (Blackburn, 2008a, b, Carruthers, 1992) that all mental states and events are physical states and events (Carruthers, 1992); they are in fact referring to the same thing (Carrier & Mittelstrass, 1995).  Two versions of the theory can now be identified, largely in response to objections from several competing theories which to an extent has divided Identity theorists.  The ‘strong’ version of ‘type identity’ claims general types of mental states will be consistent with general types of physical (brain) states, while the ‘weak’ version, ‘token’ identity[3] has the lesser claim that each individual mental state will correspond with an individual physical state.  Lowe (2004) illustrates this ‘type/token’ distinction with the word ‘tree’, which contains 4 letter tokens, but only three letter types – t, r and e (p. 48).

There are several other contenders attempting to answer the mind-body problem; to place the identity theory among its challengers, short descriptions of the main objectors are provided here.  Antithetical to the identity thesis are the various dualist theories which posit two distinct ‘pieces’ to conscious beings – a physical body and a non-physical mind.  Other criticisms come from other theories with a materialist grounding, like functionalism, which ‘characterises mental states with the causal roles they play in determining how a subject behaves in different circumstances’ (Lowe, 2004, p. 45).  A range of objections to the identity theory will now be discussed, along with how these objections are mitigated (Carruthers, 1992, Heil, 2004, Crane, 2003).

The first set of objections stem from alleged contradictions of Liebniz’s law[4]; the issues of ‘certainty’, ‘privacy’ and ‘value’.  The responses here show Carrier and Mittelstrass’ point that unacceptable results are found from ‘extreme’ use of this principle. (1995, p. 83).

Certainty is primarily a dualist objection which stems from Descartes (Lowe, 2004, p. 11); it is essentially that ‘I’ am unable to be ‘certain’ of any physical states, but ‘I’ am able to be certain of my experiences; therefore by Liebniz’s law, conscious experiences cannot be equal to brain states (Carruthers, 1992).  However, the identity theory contends that since ‘I am certain’ is clearly intentional, this use of Liebniz’s law is fallacious (Carruthers, 1992, Carrier & Mittelstrass, 1995).

The ‘privacy’ argument claims that brain states are ‘public’ (not private), but conscious states are private, therefore conscious states are not identical to brain states.  This argument has problems of its own.  Carruthers (1992) notes an inherent ambiguity in this argument, since the meaning of ‘private’ can imply two different contexts; it can be understood as either a problem of ownership, or a problem of knowledge (Carruthers, 1992, pp. 139-140).  If privacy is taken in context of ownership then the argument is unsound, as the premise claiming that brain states are ‘public’ is false, since only ‘I’ can own my brain states.  If the context of this is taken as ‘private knowledge’ then this argument suffers from a fallacious use of Liebniz’s law, similarly to the certainty argument (Carruthers, 1992, p. 139, Taylor, 2005, p. 153, Heil, 2004, pp. 80, 85).  Further, Carrier and Mittelstrass inform that the ‘privacy’ of these mental states is simply by virtue of our ‘acquaintance’ with them, and that the ‘conscious awareness’ of these mental states ‘[do] not imply knowledge’ (Carrier & Mittelstrass, 1995, p. 85).

The value argument relates to the capability of assigning ‘value’ to mental states.  The argument contends that while mental ‘conscious’ states can have value, in that they can be good or bad, no brain ‘merely physical’ states are able to have a value (Carruthers, 1992, p. 40).  Carruthers (1992) points out that a response will ultimately depend on the context of this objection, since it is questionable that ‘physical’ states cannot be subject to value claims.  If the claim is that the physical states do not have value, because it is the intent behind the physical event, then this appears to be fallacious, since it appears to use Liebniz’s law in an intentional context.

Another category of problems faced by the identity theory are qualitative aspects of consciousness.  These objections are seen by David Chalmers as constituting the ‘hard problem’ of consciousness (Heil, 2004, p. 164).  Within this category, there are two main arguments: the sensory experience of colour, and ‘felt’ sensations.  Both these ‘problems’ stem from subjective experience, which, as Heil (2004) notes, must be allowed for in a theory attempting ‘to make minds parts of a single reality’ (Heil, 2004, p. 87).  This makes the issue of qualia to be one of difficulty, and of importance to fully document in order to make the identity theory successful.

The ‘experience’ of “what it is like” (Nagel, 2005) to see “x” colour’ is seen as a problem for the identity theory, since we have no way of knowing how someone else views a colour.  We would not know what it was like for someone else to ‘see red’.  Similarly, the second problem of ‘what it feels like’ is based around the subjective nature of ‘felt sensations’; that even with ‘complete knowledge’ of the physical (brain states), we would not be able to ‘know’ how it feels to have those experiences.  This is often the basis for considering there must be something more than just our physical selves; however it should be noted that many of these issues are similarly problematical for dualist theories.

The identity thesis has several responses to these problems.  The first relates to an apparent linguistic propositional error, in which the objectors treat sensation and visualisation experiences as a particular individual thing, or a property of the thing (Carruthers, 1992, Heil, 2004).  According to Carruthers (1992), this is incorrect; since no experience has a particular colour, or a particular sensation, it is simply the way in which the mind processes visual and sensory input.  This can be a particular brain state; the key is in understanding representation, which, as Crane (2003) notes, is a problem for both materialist and dualist theories (Crane, 2003).  This representation issue is an ongoing one.  The issue of complete knowledge, however, can be responded to by understanding that the proposition is flawed; there are two types of knowledge, factual (propositional) and practical (recognitional).  Carruthers vehemently states that there is no amount of factual knowledge that can provide ability; a person must be able to apply the factual information into the appropriate field, effectively requiring the practice in having those physical states, and being able to recognise them as what they are.

The ‘qualia’ problem is related to the intentionality problem.  Conscious states are seen as unique, they are intentional or representational states, whereas no ‘merely physical’ state can be intentional in its own right.  Detractors invoke Liebniz’s law, to conclude that conscious states cannot be identical to brain states.  This is indeed problematical for the identity thesis, since the argument is valid.  However, it can be postulated that the argument may yet prove unsound, since while the intentional status of conscious states can be accepted, the soundness of the argument hinges on the acceptance of physical states intentionality.  This, however, is currently unable to be answered; though again, it should be noted that no other theory (and in particular, dualist theories) have yet been able to answer this either.

Another issue for the identity thesis is spatial position; if all mental (conscious) states, like thoughts are physical states, then they must have a spatial position.  The argument however claims that since conscious states are not attributed spatial positions, but brain states must have them, then conscious states cannot be identical.  However, there is no real reason why they could not be, with a little more research.  Carrier and Mittelstrass (1995) inform that this is simply a result from a ‘conflict between the materialist and the everyday manner of speaking’ (1995, p. 84).  The current non-use of language for this purpose is not because of an implicit ‘wrong’; it is just a ‘peculiarity of everyday speech’ (Carrier & Mittelstrass, 1995, p. 84).    If the possibility of attributing spatial positions to thoughts is accepted, it becomes possible to investigate the brain event which corresponds to those thoughts, and the location of those particular brain state changes, and then it would be possible to nominate this spatial position.

An interesting argument against the identity theory is Putnam’s Multiple Realizability argument, which claims that ‘all mental kinds are ‘multiply realized’ by distinct physical kinds’ (Bickle, 2008).  The general view of Putnam’s claims is that it puts forward against the ‘type’ identity thesis that it is conceivable that a form of life could exist in the universe which is capable of being in a particular mental state, without being in the same physical state as the identity theorist would assume (Bickle, 2008, Heil, 2004, Lowe, 2004, Ravenscroft, 2005).  The argument states that if a mental kind is multiply realizable by distinct physical kinds, then it cannot be identical to any specific physical kind – however, as the multiple realizability thesis states, all mental kinds are multiply realizable by distinct physical kind.  Therefore, it follows that no mental kind is identical to any specific physical kind (Ravenscroft, 2005, Bickle, 2008).  The identity theory has a number of responses to this argument; there is the option to follow Lewis (as cited in Bickle, 2008) in claiming that this does not remove the option of restricting claims to ‘domains’, since reductive physicalism has always been domain based (Bickle, 2008).  Alternatively, as several theorists have done, recent successes in neuroscience can be alluded to; success from neuroscience would not be expected if there was no continuity of ‘neural mechanisms’ (Bickle, 2008).  Yet we do expect some continuity across species, and have been able to apply successes from other species to humans.  This neuroscience evidence suggests that ‘psychological functions are not as radically multiply realized as … suggest[ed]’ (Bickle, 2008).  Shapiro (Bickle, 2008) also provides another option to overcome this problem – with the claim that a case of multiple realizability must be argued, since the ‘realizing kinds’ must ‘genuinely differ in their causally relevant properties’.  If there is no difference, then there is no multiple realizability; However if there is difference, Shapiro argues that they are not the same kind, and so there is no multiple realizability (Shapiro, as cited in Bickle, 2008).  In effect, the multiple realizability argument is severely weakened in its claims against the identity thesis.

The final hurdle to the identity theory is that of ‘necessary identity’.  Kripke’s identity argument (Kripke, 1981, 1983) is recognised to be a very powerful argument against materialism (Skokowski, 2007).  Kripke contends that ‘identity is not a relation which can hold contingently between objects’ (Kripke, 1981); all identities are necessary, even where they are found a posteriori; they only give the appearance of being contingent.  This is seen as a serious problem since the identity theory attempts to identify as a contingent proposition, which, in Kripke’s view, is impossible; he calls upon identity theorists to ‘explain away’ the appearance of contingency. As Papineau (2008) acknowledges, this is hard for the materialist to do, since it does in fact seem possible that the identities of mental states and physical states could have been different (Papineau, 2008).  The identity thesis has not been able to reply with a sufficiently powerful argument, but there are several responses.  Carruthers (1992) argues that identity theorists must deny the ability to successfully imagine particular feelings; the imagined states are not more than qualitatively similar (Carruthers, 1992).  Smart (Smart, 2011) responds that the ‘rigid designator’ in Kripke’s argument appears ‘highly contextual’, this rigid designator could both appear to match and appear to differ depending on the context in which we place it.  While these do not respond fully to Kripke’s argument, they do go some way to reducing its full force.

The identity thesis is far from perfect, and this essay is by no means comprehensive; space has only allowed the coverage of a brief introduction to the identity thesis, along with a superficial discussion of the main objections to the theory.  It has been shown that the essential nature of the identity thesis is that the mind and body are not distinct; this is expected to be further supported through neurological research.  The many problems faced by the identity theory were discussed; while issues of apparent contradiction of Leibniz’s law appeared easily solved, and only of minor consequence; Others, like the problems of qualia and necessary identity are highly significant, and require serious attention.  However, in spite of the problems it faces, the identity thesis is capable of responding to those objections, and provides a coherent and logical philosophical position which can explain, and assist scientific researchers with a research program.




Bickle, J. (2008) ‘Multiple Realizability’ in E. N. Zalta, (ed.) The Stanford Encyclopedia of Philosophy,Stanford University, [online], available:

Blackburn, S. (2008a) ‘necessary/contingent truths’ in The Oxford Dictionary of Philosophy,Oxford University Press, [online], available:

Blackburn, S. (2008b) ‘a priori/a posteriori’ in The Oxford Dictionary of Philosophy,Oxford University Press, [online], available:

Carrier, M. & Mittelstrass, J. (1995) Mind, brain, behavior : the mind-body problem and the philosophy of psychology, Berlin:Walter De Gruyter Inc.

Carruthers, P. (1992) ‘Mind and Brain’ in Introducing Persons: Theories and Arguments in the Philosophy of Mind,London: Routledge, 131-156.

Crane, T. (2003) The Mechanical Mind: A philosophical introduction to minds, machines and mental representation, 2nd ed., London:Routledge.

Heil, J. (2004) Philosophy of Mind: A contemporary introduction, 2nd ed., London:Routledge.

Kim, J. (2005a) ‘The identity theory of mind’ in The Oxford Companion to Philosophy,Oxford University Press, [online], available:

Kim, J. (2005b) Physicalism, or Something Near Enough, Princeton:Princeton University Press.

Kripke, S. (1981) Naming and Necessity, Oxford:Blackwell.

Kripke, S. (1983) ‘Identity and Necessity’ in S. Davis, (ed.) Causal Theories of Mind: Action, Knowledge, Memory, Perception, and Reference Berlin: Walter De Gruyter Inc.

Livingston, P. M. (2004) Philosophical History and the Problem of Consciousness, New York:Cambridge University Press.

Lowe, E. J. (2004) An introduction to the philosophy of mind, Cambridge:Cambridge University Press.

Moser, P. K. & Trout, J. D., (eds.) (2005) Contemporary materialism: a reader, London:Routledge.

Nagel, T. (2005) ‘What Is It Like to Be a Bat?’ in S. Cahn, (ed.) Exploring Philosophy: An Introductory Anthology,2nd ed.,  Oxford University Press, [online], available:

Papineau, D. (2008) ‘Kripke’s Proof That We Are All Intuitive Dualists’, London Philosophy Papers,  available:

Ravenscroft, I. (2005) Philosophy of Mind: A Beginner’s Guide, New York:Oxford University Press.

Skokowski, P. (2007) ‘Is The Pain In Jane Felt Mainly In Her Brain?’, The Harvard Review of Philosophy, 15, 58-71.

Smart, J. J. C. (2011) ‘The Mind/Brain Identity Theory’ in E. N. Zalta, (ed.) The Stanford Encyclopedia of Philosophy,Stanford University, [online], available:

Taylor, R. (2005) ‘The Mind as a Function of the Body’ in S. Cahn, (ed.) Exploring Philosophy: An Introductory Anthology,2nd ed.,  Oxford University Press, [online], available:

[1] A contingent truth is one that is not ‘necessary’; that is, it could have been different.

[2] a posteriori refers to the requirement for empirical experience to ascertain this knowledge, as oppsed to a priori knowledge which can be developed conceptually.

[3] ‘Strong’ and ‘Weak’ here are not value claims; instead they refer to a kind of purity; ‘Strong’ is the more ‘pure’ form, along with its ‘diluted’ counterpart; however this ‘dilution’ should not necessarily be considered negative.

[4] Leibniz’s law contends the identity of two entities implies the sameness of their properties – identity implies indiscernibility.

All constructive criticism is welcome! What do you think? What could I do better? Reading suggestions are welcome too!

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