The Identity Theory

Just revision notes on the identity theory

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This Theory is...

  • A Posteriori.
  • Monist (physicalism).
  • Reductionist.
  • An argument for coherence.
  • Claiming identity over correlation.
  • Revisionist.
  • Seeking to exchange correlation for identity.
  • Seeking to solve the problem of interaction.
  • Seeking to solve the problem of other minds.
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The theory was advanced by:

  • Smart.
  • Feigl.
  • Place.
  • Armstrong.
  • Carruthers

It came to prominence in the 60's and 70's, and is still held by some today.

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The original version argues that the identity claim is coherent, against those who dismissed it as incoherent.

The charge of incoherence was that by 'mind', I mean x, y, z,

By brain I mean p, q, r,

and therefore 'the mind' means one kind of thing, and 'the brain' another, wholly different kind of thing.

Identity theorists dismiss this charge as confusing the 'is' of definition and the 'is' of composition; 

Mind is not defined as a collection of brain processes - but it is composed of them.

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Smart clarifies this, invoking the distinction between sense and reference:

The identity theory is NOT to the effect that 'mind' has the same sense as 'brain', but the same reference.

'H2O' has a different sense to 'water', but the same reference.

'NaCl' has a different sense to 'salt' but the same reference. 

'Morning star' has a different reference to 'evening star' but the same reference (Venus) 


'Pain' has a different sense to 'C-fibres firing' but the same reference: a brain process

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On this view the identity of mind and brain is analogous to other contingent identity claims.

These claims cannot be established a priori, we live in a world where they just happen to be the case.

The hope was that whole classes (or types) of mental states would be indentified with whole classes of brain states.

e.g. If you are in pain you must be in brain state CFF, etc.

This contingent identity claim claim was called the type-type identity theory

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The logician Kripke objected to this claim thus:

Identity claims like: water = H2O.... salt = NaCl etc

are, if true, not contigent but necessary truths. 

(i.e. There is a possible world in which something looks like water, or salt, but is not water or salt. The matter is decided by reference to the physical essence. It may look like gold, but unless it has the atomic number 79, it is merely fool's gold)

If water is H2O, it is so necessarily, not contingently

(i.e. water is H2O in evert possible world)

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However, Kripke points out, I cannot imagine a world where pain is designated 'fool's pain' because it is not made of a certain brain tissue. On the contrary, I can imagine a world in which pain is realised (or instantiated) in many different physical ways (multiple realisation)

But if ALL identity claims are necessary, and mental states are NOT necessarily brain states, then the identity claim must fail totally.

He implies that some form of dualism must therefore be true.

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McGinn replies that the identity theory can be saved from this criticism, by diluting the claim from a type to a token theory.

The claim:

This mental state is necessarily identical with this physical state, seems to preserve both identity and multiple realisation.

(A clock can be made of wood, water, clockwork, quartz, stone, etc. If you have one of these physical structures you necessarily have a clock. But to have a clock, you don't need clockwork.)

This is called the token-token identity theory - The identity theory is now necessary, not contingent, and ranges over particulars, not classes. 

Being in a mental state supervenes on physical factors. If I am in brain state CFF then I am necessarily in pain, but if I am in pain it may be instantiated in various different ways.

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Putnam points out that it is the functional, not the material essence of the mental state that now wears the trousers.

The essence of a heart is that it pumps: we are indifferent as to the stuff it is made of. Similarly, what matters about a mental state is its function. Reflecting on multiple realisation makes us understand that the physical issue is not important.

As Descartes had indicated, the essense of a mental state is different from the essence of a physical state.

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So, in so far as identity theory wants to defend a strict identity of mind = brain.  It cannot be true.


the CD = music

Computer = the program

the £10 = the £10 note

We have here two essences, not one; a functional and a physical essence, and they are distinct. This explains why any attempt to identify these things ends in failure: they cannot share all the same properties as they are different things.

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Another criticism attacks the original analogy with scientific identity statements.

These statements are unproblematic as to the relation of the essence and the attributes. The essense explains the emergence of say, liquidity, or having a crystalline structure.

But how can any reference to a physical essense explain the emergence of pain, memory, imagination, etc.?

This is the explanatory gap.

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