Multiple Realizability and Mind-Body Identity

It is generally held that type-identity theories of mind have been definitively discarded by Putnam’s multiple realizability argument and by Kripke’s thesis on necessary identities. My general goal is to challenge this opinion, even if under some conditions, and to provide an argument in support of a causal reading of sensations that will deflate the importance of their individuation via qualitative properties. The multiple realizability argument is generally taken to show that identity statements between mental properties (say, have pain) and their realizers (C-fibers firing) are not necessarily true. These are contrasted, famously by Putnam and Kripke, with statements such as “heat = molecular motion”, which are considered as necessarily true. However, I will argue, the latter identity statement is subject to the same kind of multiple realizability. Many authors have already noticed that there are many ways in which the supposed identity “heat = molecular motion” may be shaken. On the one hand the concept of heat can be applied to different states of the matter (gases, plasma, vacuum) and in some of these cases the supposed identity with molecular motion is no longer necessarily valid. On the other hand, inter-level identities allow for minimal variability: two objects having the same temperature may have different physical arrangements of moving molecules. Both these observations, though, do not exclude that the identity holds at least in some specific phase of the matter (say, gases). I want to argue that also in this case the supposed identity could be nevertheless multiply realized, and in a more serious way then individual variability. The main hallmark for having multiple realizability, and not just multiple instantiation, is the presence of different natural kinds fulfilling a given high order property. In this sense, heat is multiply realized by molecular motion because it can be realized by different kinds of molecules, which are different natural kinds. This shows that the supposed identity “heat = molecular motion” is nothing more than a schema of identification. In order to obtain an identity statement it is necessary to fill the logical form of the schema by introducing co-referential rigid designators on both sides of the identity sign. Once this is done, we can have necessarily true identity statements again, but these have a quite narrow scope of validity. The same reasoning can be applied in the case of the supposed identity between pain and C-fibers activation or, more in general, between mental states and physical states. In such case, we have to narrowing the scope of the physical realization conditions of the mental state or property in the same way in which this is done in the case of purely physicalistic statements. Once this is done, identity statements relating mental and physical properties are on the same boat of those concerning physical properties alone. In the second part of the paper, I argue that the way in which the multiple realizability argument is tackled with respect to identity statements on physical entities (heat = molecular motion) can be applied also to identities relating mental and physical properties. I argue that the previous strategy not only provides an answer to Putnam’s argument but that, if supplemented, blocks Kripke's intuition according to which pain states find their identity conditions in the phenomenological component of such sensations. To this end, I analyze what sensations are and what their phenomenological component is. I first maintain that sensations are stable relations with properties of the world fixed by token-reflexive conditions of the receptors. Secondly, in order to individuate their qualitative component, is sufficient to consider their distinctiveness, leaving any qualitative consideration apart. Applying the above analysis to the case of pain we notice that it fits well with the distinction found in the medical literature between feeling pain and detecting pain. The detection can be individuated in purely causal-functional terms, on the stability criterion, while the feeling is conveniently considered in evolutionary way, on the distinctness criterion. Such an individuation may be not metaphysically necessary but such a strong reading of necessity is not what is needed for our goals. The general upshot is that the identity thesis, as originally proposed by Smart, Place and others, is no longer viable. In its place we should introduce more narrow tailored identities, but these are not different from those that we should accept in case of purely physicalistic terms, such as heat and molecular motion. Having set all this, I conclude that the type-identity theory of mind can be vindicated.