The Mind/Brain Identity Theory:
A Critical Appraisal

2. The Problem of Phenomenal Properties

Book cover: The Mystery of Consciousness by John R. Searle

The problem that I wish to begin with derives from the application of the law of indiscernibility of identicals (Leibniz's Law). This law states that if two items are numerically identical, then for any property, it is a property of one if and only if it is a property of the other. Formally,

x = y(F)(FxFy)

The identity theorist, in identifying mental items with physical items, means by 'identity' this notion of strict numerical identity.

The objection to making this strict identity is that there is a class of properties, the phenomenal properties, that do not satisfy the law of indiscernibility of identicals. Phenomenal properties can be roughly categorized as those that can be ascribed directly to sensations, but only indirectly to items that are not sensations. These phenomenal properties, the objector claims, are easily applicable to mental items. But, they argue, it is either meaningless or false to attribute these same properties to the physical items, such as brain states, that identity theorists wish to identify with mental items.[4] It makes sense, they continue, and is sometimes true to say that we have red, round, loud or sweet sensations, tingling, burning, sharp or severe pains, and happy, angry or nostalgic feelings.[5] But it is either meaningless or false to say that we have red, round, etc., brain states.[6]

The reply that I want to make in defence of the identity theorist is that phenomenal properties cannot strictly be construed as properties of sensations. This response obviates the application of the law of indiscernibility of identicals in this instance. The surface structure of the common idiom, 'I have a red sensation', can fool us into thinking that there are sensations that are red. But sensations are not coloured. There is not another set of colour receptors lying behind our sensations that can discriminate their colour. We do not see sensations. The idiom, 'red sensation', is simply a colloquial locution of the more accurate phrase, 'sensation of redness'.

These same comments apply to the locutions 'round sensation', 'loud sensation' and 'sweet sensation'. Sensations do not make noises, nor can they taste like sugar. Nor do they come in various shapes and sizes.[7] Once again, the same can be said for 'happy', 'angry' and 'nostalgic feelings'. Literally speaking, it is not our feelings that are happy or angry or nostalgic, but ourselves. And our feeling happy or angry or nostalgic is constituted by the fact that we have feelings of happiness or of anger or of nostalgia.[8]

The case of pains is a little more complicated. However, I think that we can analyse pains in the way we do colours. In that case, the sentence, 'I have a sensation of pain' becomes logically akin to 'I have a sensation of colour'. The term 'burning', then, in the report, 'I have a sensation of a burning pain', modifies the term 'pain' and not the term 'sensation', just as the word 'red' in the report, 'I have a sensation of a red colour', modifies the term 'colour' and not the term 'sensation'. So, just as 'red' is not a property of sensations, neither is 'burning'. (The colloquial locutions, 'I have a burning sensation', and 'I have a painful sensation' can easily be treated in the same way as I have treated 'red sensation' above.) I think that we can now treat 'red after-images' in the same way as I have treated 'burning pains'. So, 'I am having a red after-image', can be more accurately rendered as, 'I am having a sensation of a red after-image'.

The notion of 'having' that occurs in reports such as, 'I have a sensation of redness', is no more problematical than the same notion that occurs in reports of physical states of our bodies, such as, 'I have a head of hair'. If the mind is the brain, as the identity theorist supposes, then what is reported when we report states of our mind will in fact be numerically identical with states of our brain.

I think this relatively simple semantic analysis of sensation reports is true independently of the truth or conceptual coherence of the identity theory. If this is so, then it cannot be a valid criticism of a theory that identifies sensations with brain states that on opening up people's brains for inspection we do not find red or burning or angry brain states.[9]


  1. [4] Direct realists and phenomenalists will strongly disagree with this rough definition, but this will not matter for my purposes here. For those who disagree, the phenomenal properties can be extensionally indicated as colour, pitch, taste, smell and feel.
  2. [5] I am aware that it has been seriously disputed whether this last class of items, namely feelings, is a class of sensations. Nonetheless, I have included it here for the sake of completeness.
  3. [6] For an example of the 'phenomenal properties' objection, see Rollins [1971: 27]. Here, he says, 'Prima facie, no language would be complete for describing experience if it did not allow us to speak of intensity, dreariness, nostalgia, a sense of familiarity, awe or overwhelmingness, placidness, or, in general, of something occurring to us which is of such a nature as to neither have nor lack a locus. In our understood language, descriptions like these could not be eradicated without loss of descriptive power necessary for the language of experiences; yet none of them, of course, can intelligibly be predicated of a brain process itself.'
  4. [7] It should be noted that 'round' is not strictly a phenomenal property, for it can be directly ascribed to items that are not sensations. I include it here simply for the sake of completeness.
  5. [8] Here, I think J. J. C. Smart [1971: 87], in defending the identity theory, had made a tactical error in replying to Rollins' 'phenomenal properties' objection (see footnote 7 above) by admitting that 'some brain processes are nostalgic'. I have never met an experience, and hence a brain process, that was homesick. Experiences do not even have homes to be homesick about.
  6. [9] The 'phenomenal properties' objection can receive replies within J. J. C. Smart's topic-neutral account of sensation reports and D. M. Armstrong's Causal Theory of Mind. See Smart [1977: 59–61] and Armstrong [1968: 116f]. To the extent that these replies differ from the one that I have given above, I cannot concur, for I consider Smart's and Armstrong's translations of sensation reports to be seriously mistaken (see below). J. Cornman has offered the interesting suggestion that the law of identity of indiscernibles does not apply to 'cross category' identities. Whether Cornman's thesis is true or not, I do not believe that it is relevant in the case of sensations, for I have argued here that sensations do not have phenomenal properties. See Cornman [1977: 123–9].

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