By Robert Kirk
Physicalists like me think everything in the world is ultimately physical, and that the physical facts provide for all the facts, including consciousness. But how should we conceive of the link between physical and mental? It is helpful to think in terms of descriptions. Descriptions in the vocabulary of physics can specify everything physical that there will have been; and physicalism commits us to the view that truths in that vocabulary specify everything that there will have been. But those truths use a very restricted set of concepts. What about truths in terms of different conceptual schemes, for example those of other sciences such as astronomy, biology, and psychology; or of more or less unscientific specialities such as history and art; or of everyday language? Physicalists must hold that those truths are redescriptions of a reality describable in terms of the narrowly physical vocabulary: of parts or aspects of the same vast swirl of matter and energy. A pebble is an aggregate of atoms and subatomic particles; if you have the aggregate, you have the pebble. So if we say there is a pebble at a particular place and time we are redescribing what could have been specified in terms of the locations and states of particles. Similarly (we physicalists must hold) mental truths such as ‘Victoria loves Albert’ are redescriptions of parts or aspects of what could have been specified – though much less intelligibly – in terms of the locations and states of particles.
Although those points are familiar, it is not generally recognized what they imply about the physical-to-mental connection. Saying it is necessary is not enough: even dualists can accept that. One popular approach is inspired by Kripke. According to ‘a posteriori physicalism’, physical truths necessitate mental truths on account of a posteriori necessary psycho-physical identities. Unfortunately this approach doesn’t work if we take those points about redescription seriously. If mental truths are redescriptions of a reality specifiable in narrowly physical terms, then the physical truths (I regret having to use this expression) ‘logico-conceptually entail’ truths stated in terms of non-physical vocabularies: it is impossible for broadly logical and conceptual reasons that those narrowly physical statements should have been true and those non-physical statements false. Although this logico-conceptual entailment thesis allows physicalism to be only contingently true, it contrasts sharply with the usual kind of a posteriori physicalism, which is not even sufficient for physicalism (and is actually inconsistent with it if I am right).
You might object that there are no analytic connections from physical truths to truths about experiences. But semantic rules can determine that a given description specifies a certain item and also that this item qualifies for a certain redescription. That means there can be logical and conceptual links between the descriptions as wholes even if there are no analytic links.
In opposition to the Kripke-inspired a posteriori physicalism mentioned earlier, David Chalmers and Frank Jackson argue that physicalists should maintain that mental truths are derivable a priori from the totality of narrowly physical truths. That approach differs significantly from the one I recommend, and I think it too is mistaken. Physicalists must maintain that while the link from physical to mental is logico-conceptual, it is not necessarily knowable a priori.
That may strike you as paradoxical. How can phenomenal truths be logico-conceptually entailed by narrowly physical truths if not even a being with superhuman cognitive powers could infer them a priori from the latter? But logico-conceptual entailment depends solely on logico-conceptual facts, while a priori inferrability depends additionally on epistemic facts: a crucial difference. The logico-conceptual entailment thesis is compulsory for physicalists, but there are no good reasons why they should also hold that the facts about what our experiences are like can be inferred a priori from the narrowly physical facts. Indeed there are powerful reasons to the contrary, as vividly illustrated by Jackson’s famous example of Mary. You may suspect that all this only brings out an inconsistency at the heart of physicalism, but it doesn’t.
The key consideration is that any satisfactory explanation of consciousness will include an explanation of our special epistemic position with respect to our phenomenally conscious states. It is only by actually having experiences that we learn what it is like to have them; and we do that without having to know the underlying physical or functional facts. We have developed special concepts for describing experiences from our own point of view even if we are ignorant of those underlying facts; which explains a sense in which phenomenal concepts float free of physical and functional concepts. So (I argue) physicalists can consistently endorse the logico-conceptual entailment thesis while denying the a priori entailment thesis.
That conclusion depends on the controversial assumption that there can be a physicalistically acceptable explanation of consciousness. Many people are so impressed by intuitions about zombies and transposed qualia that they believe that merely physical truths cannot logico-conceptually entail phenomenal truths, in which case physicalism and functionalism are sunk for that reason alone. I think they are wrong, but I have not been attempting to defend physicalism here, only to suggest how physicalists should conceive of the relation between physical and mental truths.
Robert Kirk is emeritus professor of philosophy at the University of Nottingham, where he worked for 33 years. His first degree was in classics, but he was inspired to switch to philosophy by reading Quine’s Word and Object. Kirk’s books include Relativism and Reality: a Contemporary Introduction, Zombies and Consciousness, and his latest work: The Conceptual Link from Physical to Mental (OUP, 2013).
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