For a time 50 years ago the contingent identity thesis was all the rage in the philosophy of mind. The idea was simple. While mental terms and physical terms (brain language) did not mean the same thing, mental and brain language could still refer to the same thing. Just as 'Morning Star' and 'Evening Star' mean something different - - they have different modes of presentation according to Frege - - and yet nonetheless refer to the same object Venus, so could a mental term (e.g., 'headache') and a brain term (e.g., 'C-fiber stimulation of such-and-such a variety) mean something quite different, yet refer to the same physical state of affairs. The idea was that just as there are possible worlds where water is not H2O, there are possible worlds where pain is not C-fiber stimulation. (This world is apparently the actual world. I use 'C-fiber stimulation' here because this is what was used in the literature.)
According to the contingent identity thesis, as I read it, there must be some criterion of trans-world individuation for, respectively, water, lightning, and pain states that does not include, respectively, H2O, electrical discharge, and brain states. One is left to puzzle out exactly what this might be, the contour of the "is of definition," but this much is clear: If it only happens to be the case that water is H2O, then being H2O is only accidental to water. There must be something about water that allows projection across possible worlds, something that makes water water in all of these worlds --whether or not these worlds even have molecules at all. A fortiori, there must be something about the phenomenological state of pain that makes pain pain, regardless of how that pain is realized in neurostates - - or even if it is realized in neurostates. (I can clearly make sense of the notion of an angel being in pain, yet angels by their nature are incorporeal, and thus could not have brain states at all.)
The contingent identity thesis seemed to be committed to a Lockian view of secondary qualities. Heat is whatever is in the object that, in the presence of a suitable percipient, would cause the sensation of heat in the subject. The thesis thus presupposes a distinction between heat, the experience of heat, and the atomic facts which are identical to the heat in the object. The important point here is that heat is an objective property; its criterion of transworld individuation is not subjective, but rather an objective fact about the thing which has the power to produce an experience of an appropriate kind in the subject. On this view, heat just is that power in the object capable of producing the sensation of heat in the subject, and it is this power that is contingently identical to mean molecular kinetic energy.
This view rather nicely deflated phenomenological ontology, according to its advocates; the experience of heat was not a thing that was being experienced, but rather the experiencing of the thing. Adherance to the adverbial theory of perception was celebrated, for one did not wish to reify phenomenological content, but rather to speak about the thing that was experienced phenomenologically. Heat is in the thing, and the phenomenon of heat is just my experiencing of heat in such-and-such a phenomenological way.
The problem was, of course, what to do with putative phenomenological objects that seemed to have being apart from any external situation. What about an after-image? If I see a green after-image, I am not seeing that power in the object capable of producing an experience in me of a particular type, but rather am just seemingly experiencing the phenomenological object itself. The question is this: What is the being of an after-image? The answer that Smart and Place gave was that an after-image just is a brain process of a certain way. A green after-image is the experience of something that would be normally associated with the experience of a green object. Green, like heat, is a power in the thing which can produce an experience in us. A green after-image is the experience normally associated with the presence of a green object.
The problem with the contingent identity thesis, among other things, was and is that there are no contingent identities. As Saul Kripke argued so forcefully in Naming and Necessity, all identities are necessary. If a =b, then a just does not happen to be b, a is b. There are no possible worlds in which a thing is not identical to itself. If the Morning Star is the Evening Star, and both 'Morning Star' and 'Evening Star' designate Venus, then because 'Venus = Venus' and there is no possibility that 'Venus is not Venus', then 'Morning Star is Evening Star' is true in all possible worlds. The idea here is that names and expressions form rigid designators. Transworld identification is not done on the basis conceptual content, but by the initial baptism of the name to the thing. Once we discover that the Morning Star = Venus = the Evening Star, then we cannot think the Morning Star not being the Evening Star, because to think such a thing is not to think of the Morning Star at all. (It would be to think of something very much like the Morning Star' except for it not being the Evening Star.)
According to Kripke the apparent contingency of 'heat = mean molecular kinetic energy' is due to the fact that we think that it could have been otherwise. We did not know the heat is mean molecular kinetic energy so we naturally think there are possible worlds in which it is not. But obviously, if this is what heat is, then heat could not have been something other than mean molecular kinetic energy. This is not an example of what Place called the "is of constitution," but rather of what he called the "is of definition." The human race simply did not know for most of its history what heat was, but that does not mean that it was somehow not necessary that heat was mean molecular kinetic energy. One's epistemic limitations with respect to necessary truths do not infect the modal status of those truths.
Kripke thus distinguishes, like the materialists, from among the heat, the molecular realization of the heat and the experience of heat. There are qualitatively indistinct experiences that are experiences of different things. The contingency comes in the relationship between the percipient and the thing with heat, not in the relationship between heat and what heat ultimately is. Similarly, green just is a certain range in the electromagnetic wave spectrum, although we might have an experience of that which is qualitatively indistinct from it but not yet be an experience of green. (Presumably, this is the experience of a green after-image.)
What Kripke forcefully points out, however, is that the materialists are simply wrong if they believe that a mental state just is a brain state. The mental state of experiencing green cannot simply be a brain state as they argued, for there is a fundamental disanalogy between the example of 'lightning is an electrical discharge' and 'pain is C-fiber stimulation of such-and-such variety. In the first we can distinguish the lightning, the electrical discharge and are experience of lightning. There is a contingency between the experience of lightning and the lightning that just is not present between the pain and the experience of pain. If the lightning is necessarily identical to electrical discharge, our explanation for not knowing that is our failure to have gotten clear on whether our experience of lightning was actually of lightning. If we have the presence of lightning, we have the presence of electrical discharge in all possible worlds. The explanation of why we did not always know this, is that we were not clear on the identification of lightning.
This explanation is not, as Kripke points out, available to the one seeking to identify pain with C-fiber stimulation. The reason for this is that there is no meaningful distinction between pain and our experience of pain. If pain is necessarily C-fiber stimulation, then we have no way to explain the apparent contingency between pain and such stimulation. While we can imagine possible worlds in which we could have an experience of heat that is not mean molecular kinetic energy - - because our experience of heat is turns out not to be of heat - - we cannot imagine possible worlds in which we have an experience of pain that is not mean molecular kinetic energy. We accordingly should not be able to entertain the possibility of types or tokens of mental states that are not types or tokens of physical states and, alternatively, types or tokens of brain states that are not types or token of mental states. But we can, as Descartes has shown us, easily think the possibility. If the only identity available between mental states and brain states is necessary identity, then it seems that Descartes' argument has new legs.
Start from the indiscernibility of identicals, the assertion that anything identical to itself has all properties in common. If pain states and brain states do not have all properties in common - - do they have anything in common except temporal position? - - then they simply can't be the same thing. If there is no possiblity that the terms can mean something different yet be the same thing, then there is no possibility that the mental and the physical are the same thing.
Now Kripke is not a Cartesian substance dualist, and he wants very much to protect himself from the application of such an appellation. However, it is true that his work on the nature of identity continues to give ammunition to those uneasy with facilely naturalizing the mind. This is especially true when it comes to the qualia of the mental, the raw feels of color and pain experience. How can it be true that pain is identical to some brain state or other, if the identity for the person is necessary? How finally does the intuitive counterfactual that "I could experience this even after death" get answered if the 'this experienced' is supposed finally to be necessarily identical to some brain state or other? This question remains, and it is not without theological significance.
There is no reason why Lutheran theology should not be interested in such questions as these. I continue to believe that tacit philosophical assumptions must be coaxed out and explored, if theology is profitably to address the questions of the contemporary intellectual horizon. I can see no more pressing question than that of the ontological status of mind.
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