How Many Explanatory Gaps Are There? E. Diaz-Leon University of Manitoba According to many philosophers, there is an explanatory gap between physical truths and phenomenal truths. Someone could know all the physical truths about the world, and in particular, all the physical information about the brain and the neurophysiology of vision, and still not know what it is like to see red (Jackson 1982, 1986). According to a similar example, someone could know all the physical truths about bats and still not know what it is like to be a bat (Nagel 1974). We can conceive of an individual that is physically identical to me, molecule per molecule, but does not have any phenomenally conscious state whatsoever (Chalmers 1996). Some philosophers have argued that the explanatory gap shows that we cannot explain consciousness in physical terms (Levine 2001), or even that phenomenal consciousness is not physical and therefore physicalism is false (Chalmers 1996, 2002). Gilbert Harman has recently suggested (Harman 2007) that we can see the explanatory gap regarding consciousness as a particular case of a more general phenomenon. He claims: ‘A somewhat different explanation of the explanatory gap appeals to a difference ... between two kinds of understanding, objective and subjective. Objective understanding is characteristic of the physical sciences ... These sciences aim to discover laws and other regularities involving things and their parts, in this way achieving an understanding of phenomena “from the outside”’ (2007: 1). On the other hand, he adds, subjective understanding is characteristic of the sciences of the mind broadly conceived, such as social and psychological sciences, which are concerned with phenomena that cannot be completely explained within an entirely objective approach, and require an understanding “from the inside”. For instance, he claims that ‘there are aspects of reasons, purposes, feelings, thoughts, and experiences that can only be understood from within, via sympathy or empathy or other translation into one’s own experience’ (2007: 2). Harman discusses several examples of this explanatory gap between the objective and the subjective aspects of reality. In what follows, I will discuss these examples and examine whether the explanatory gap between the physical and the phenomenal can 1

really be seen as a phenomenon of the same sort, or whether it has any special features that require special treatment. I will offer some motivation for this second option. The first example concerns intentionality. Harman argues that a purely objective approach to the behaviour of a certain community could not tell us, say, what certain morning rituals that certain individuals are performing mean (Harman 2007: 2). Indeed, a purely behavioural description of those activities would not be enough for finding out what they are doing, that is, what sort of actions they are. There is some additional information that is missing, namely, information regarding the mental states of the individuals that perform those movements. If we had sufficient information concerning the content of their beliefs and desires (and perhaps other mental states), we would be in a position to know whether they are performing a religious ritual, practicing a dance, or just doing some morning exercise. However, I contend, it is widely accepted that a purely objective approach would be able to provide a complete account of the content of their beliefs and desires. To illustrate that point, we could follow Chalmers’ distinction between the psychological and the phenomenal aspects of the mind (1996, Ch. 1). The psychological aspect has to do with those properties of mental states that are independent from consciousness, and which, plausibly, can be explained in physical-functional terms, whereas the phenomenal aspect of the mind has to do with what it is like to undergo conscious mental states (and it is more controversial whether those features can be explained in physical-functional terms). Chalmers, among many others, considers intentionality to be a psychological aspect of the mental, since it seems very plausible to think that we can explain intentionality in physical-functional terms. If this is so, then there are no good reasons to think that a purely objective approach is not enough for explaining actions. Harman discusses a second example of the gap between objective and subjective understanding, namely, the problem of providing a translation of the language of a certain group of people into one’s own language. In this case, he argues, it seems that a purely objective account would not be enough either. If we want a correct translation of a certain language into our own language, two additional requirements will be needed:

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First, one must be able to identify a certain objectively described language as one’s own language, an identification that is itself not fully objective. Second, one must have in one’s own language some expression that is used in something like the same way as the expression in the other language. (2007: 3)

That is to say, if there is some expression E in the original language for which there is no expression E* with a similar function in my own language, then I cannot provide a translation of E into my own language. Using this analogy, Harman has offered a possible solution to the problem of explaining the experiences of other creatures. Let’s see how it works: suppose that we could draw a correspondence between the possible experiences of one creature, say, a bat, and the possible experiences of another creature, say, a human being (where this correspondence is described in a purely objective manner, for instance, in terms of physical-functional relations). In order for this objective account to provide an account of what it is like to be a bat that we can understand, two additional criteria have to be satisfied: (i) we have to identify the second system of experiences as our system of experiences, that is, as the class of possible experiences that we can have; and (ii) for any possible experience of the bat, there must be an experience of ours that is functionally similar to it (2007: 3). According to this model, how could Jackson’s Mary know what it is like to see red? First of all, she would need an account that draws a correspondence between the experiences of normal perceivers when they see red, and her own possible experiences (in purely objective terms). In addition, she would need to have had an experience of seeing red herself. Otherwise, she could not know what seeing red is like. And, crucially, this is something that a pure objective account cannot provide: this is where a subjective approach is needed, according to Harman. Why is it the case that in order to know what seeing red is like, you must have had the experience of seeing red? Because, we can assume, in order to know what seeing red is like, you need to be able to entertain the relevant thought. As Harman says: ‘Knowledge that P requires being able to represent its being the case that P’ (2007: 4). And in order to entertain propositions about what it is like to see red, you need to

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possess the constituent concepts. One crucial concept is the phenomenal concept ‘red experience’, which arguably you can possess only if you have had that experience. I agree that this model can explain the existence of an explanatory gap in the case of experiences that we have not had ourselves or that we cannot imagine having. It is pretty clear that a physical account of the world will not put us in a position to know what certain organisms are experiencing, if we have not had those experiences before, because in that case we will not possess the required concepts. However, I think that even when we solve this problem, there is still an important explanatory gap remaining. This further explanatory gap can be stated as follows: Imagine someone who possesses all the relevant concepts, both physical and phenomenal. This person could know all the physical truths about the world, and still not be in a position to know what experiences someone is having, say, that I am feeling pain at time t. This is not due to the fact that this subject does not possess the concept ‘pain’, because by hypothesis she does. The problem is that she would not be able to infer a priori the phenomenal truth that I am feeling pain at t from a complete physical description of the world, since, arguably, physical truths do not entail phenomenal truths a priori (Chalmers 1996, Ch. 3). In my view, we can distinguish two different explanatory gaps concerning consciousness, which require different treatment. On the one hand, we have the (“easy”) explanatory gap which manifests itself in examples such as Nagel’s bat example or Jackson’s Mary thought-experiment: the idea here is that a complete physical description of the world does not necessarily put someone in a position to know what certain experiences are like. This is due to the fact that we might lack the relevant phenomenal concepts. On the other hand, there is a remaining explanatory gap which manifests itself in the fact that knowing a complete physical description of the world would not put us in a position to know what different experiences different organisms are having, even if we possess phenomenal concepts to refer to all these experiences. 1 The problem is that such a physical description of the world would not entail a priori a phenomenal description of the world. The explanatory gap in this second sense, that is, the fact that a complete physical description of the world (P) would not entail a priori a phenomenal description of the

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world (Q) is invoked by recent formulations of the conceivability argument against physicalism. For instance, Chalmers (2002) formulates the argument as follows:

(1) It is conceivable that P&⌐Q. (2) If it is conceivable that P&⌐Q, it is metaphysically possible that P&⌐Q. (3) If it is metaphysically possible that P&⌐Q, then materialism is false. (4) So, materialism is false. (2002: 249)

The first premise of this argument claims that P&⌐Q is conceivable: by this, it is meant that we can conceive of P&⌐Q without contradiction, that is, it is not a priori false. Or in other words, P→Q is not a priori true: this corresponds to the second characterization of the explanatory gap that I have sketched above, and this is what is supposed to entail that P→Q is not metaphysically necessary and therefore materialism is false. The question now is: could we use an explanation along Harman’s lines in order to solve the problem posed by the conceivability argument? I think that it is clear that we cannot, and the reason is the following: The problem posed by the explanatory gap in this second sense (that is, that P→Q is not a priori true) cannot be solved merely by saying that we are not in a position to know P→Q a priori because we fail to have some phenomenal concepts. Crucially, we are also unable to infer Q from P a priori even when we possess all the relevant concepts, both physical and phenomenal, (or so the first premise of the conceivability argument says). Therefore, in order to answer the challenge posed by this argument, we need to either show that in fact we are really able to infer Q from P a priori, or argue that even if P→Q is not a priori true, that does not entail that it is not metaphysically necessary. And, to the best of my knowledge, Harman’s model does not accomplish (and does not even aim to accomplish) any of these two tasks. What Harman’s model does provide is a very plausible response to Jackson’s knowledge argument and Nagel’s bat argument: it can be argued that the reason that Mary could not learn what it is like to see red from her black-and-white room, or the reason that we cannot know what it is like to be a bat, is that we lack the relevant 5

phenomenal concepts given that we haven’t had the corresponding experiences (and this is perfectly compatible with materialism). However, as we have seen, the conceivability argument sketched above poses a harder challenge for physicalism, which still has to be answered satisfactorily. 2 3

References Chalmers, D. (1996) The Conscious Mind, Oxford: Oxford University Press. Chalmers, D. (2002) “Consciousness and its Place in Nature”, in D. Chalmers (ed.) Philosophy of Mind: Classical and Contemporary Readings, New York: Oxford University Press: 247−72. Harman, G. (2007) “Explaining an Explanatory Gap”, APA Newsletter on Philosophy and Computers 6/2. Jackson, F. (1982) “Epiphenomenal Qualia”, Philosophical Quarterly 32: 127−32. Jackson, F. (1986) “What Mary Didn’t Know”, Journal of Philosophy 83: 291−95. Levine, J. (2001) Purple Haze, Oxford: Oxford University Press. Loar, B. (1997) “Phenomenal States (revised version) ”, in N. Block, O. Flanagan and G. Guzeldere (eds.) The Nature of Consciousness, Cambridge, MA: MIT Press: 597−616. Nagel, T. (1974) “What Is It Like to be a Bat?”, Philosophical Review 83: 435−50. Stoljar, D. (1995) “Physicalism and Phenomenal Concepts”, Mind and Language 20: 469−94. 1

Stoljar (2005) makes a similar distinction.

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For the record, I do think that conceivability arguments can be answered satisfactorily, from the physicalist point of view. A very persuasive response is the so-called “phenomenal concept strategy”, which argues that we can explain the fact that P→Q is not a priori, in terms of the special features of phenomenal concepts. Loar (1997) is an excellent exposition of this view. 3

I am grateful to Peter Boltuc for very useful comments.

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How Many Explanatory Gaps Are There?

In what follows, I will discuss these examples and ... However, I contend, it is widely accepted that a ... someone is having, say, that I am feeling pain at time t.

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