On Explaining Phenomenal Consciousness

Franz-Peter Griesmaier

Department of Philosophy

University of Wyoming

Laramie, WY 82071

Abstract

Over the last two decades, doubts have been expressed about the adequacy of materialism as the correct framework for explaining phenomenal consciousness (the experience of saturated greenness one has when looking at a lush lawn, for example). In this paper, I reconstruct a generic form of the various arguments that have been used to defend the view of the materialistically inexplicable nature of consciousness, or MINC. This reconstruction reveals that the arguments turn on an impoverished notion of explanation. By discussing some examples from the history of science, I show that a reasonable notion of explanation has to be wider than the one utilized in the argument for MINC, which opens the possibility for the materialistic explicability of consciousness. I end the paper by raising a question about the very intelligibility of the project of trying to explain the so-called nature of consciousness as opposed to the regularities characteristic of the relation between states of consciousness and brain states.

1. Introduction

Over the last two decades or so, doubts about the adequacy of materialism as the fundamental philosophical framework for pursuing questions about the mind have found increasingly bolder expressions. Drawing their inspirations from Thomas Nagel (1974), various authors have launched arguments against materialism that center around the "mystery" of phenomenal consciousness. It has been alleged that the feel1 that accompanies various mental states, such as the fruity tartness I experience when sipping a glass of Beaujolais Noveau, cannot be reductively explained in materialistically acceptable categories. Since Joseph Levine's (1983), this move has become known as the argument from the explanatory gap, and David Chalmers relied heavily on it in his (1996) to argue for the falsity of materialism. More recently, the focus of the debate has shifted to the question as to whether the explanatory gap is forced upon us by the necessary failure of any attempt to provide a conceptual analysis of phenomenal consciousness in physical or functional terms (Block and Stalnaker, 1999; Chalmers and Jackson, 2001). Instead of pursuing the issue further along this more recent line, I propose, in this paper, to back up one step and address the question as to whether the alleged explanatory gap really exists.

Given the dialectical role of the explanatory gap as a weapon against materialism, it is important to begin by pointing out that phenomenal consciousness is allegedly inexplicable from a materialistic perspective. Let me call this thesis MINC, short for the thesis of the Materialistically Inexplicable Nature of Consciousness. If MINC is correct, and if consciousness and especially phenomenal consciousness are undeniable and robust phenomena, it follows indeed that materialism, construed as the claim that every phenomenon either is physical itself or otherwise supervenes on physical phenomena (and thus can be explained in physical terms), is false. I won't discuss the second premise of this argument, but instead focus on the reasons that have been advanced in favor of MINC. In section 2, I will reconstruct a generic argument for MINC, which I shall criticize in section 3. The concluding section 4 will raise a question about the very intelligibility of the project of understanding the so-called nature of consciousness and end with some preliminary remarks on how alternative models of explanation might eventually help us in understanding consciousness. Those remarks will be a far cry from even the outline of a research programme. Thus, someone interested in finally understanding consciousness will be disappointed by what follows. However, someone who doesn't want to give up on materialism despite the difficulties consciousness poses might possibly find some reason for hope.

2. A Generic Argument for MINC

In this section, I plan to unearth the common structure of the arguments for MINC in order to bring to light two assumptions about successful explanations that, while making MINC initially plausible, are ultimately untenable. As a first step, consider the following ways in which the "hard problem" about the mind, namely, the nature of phenomenal consciousness, has been characterized in the literature. Chalmers (1996: 47) claims that 'even if the appropriate functional organization always gives rise to consciousness in practice, the question of why it gives rise to consciousness remains unanswered'. In other words, even on the optimistic assumption that we may come to have complete knowledge about the correlations between brain states and states of consciousness, we still would 'want to know how the correlated brain states do the job of generating and implementing states of consciousness'. (Seager 1999: 18) Michael Tye (2000: 22) characterizes the problem similarly: 'One who has a complete understanding of the term 'pain', for example, and is fully apprised of the physical facts as we know them, can still coherently ask why such-and-such brain processes or functional states feel the way pains do or why these processes feel any way at all'.

These remarks suggest that the "hard problem" is that of understanding why and how brain states give rise to, or generate, experiences. In fact, those two questions ought to be treated separately, as both illuminate different aspects of the puzzle and its alleged unsolvability. Consider, first, the question of how brain states give rise to phenomenal states. This question asks for the specification of some mechanism that is responsible for the generation of experiences on the basis of certain brain states or processes. Thomas Nagel might have been the first one to call into question the intelligibility of the project of finding such a mechanism. He argued in (1974) that purely physical and functional descriptions of an entity's mental life necessarily leave out the subjective, perspectival character of the experiences that accompany that mental life. Thus, it seems that there is no hope for finding a mechanism that could mediate between physically specifiable brain processes and only experientially accessible, and thus subjective, phenomenal states. Although we can imagine mechanisms that take brain processes as their input, we can't imagine mechanisms that produce qualia as their output, for the simple reason that qualia, with their intrinsic subjective character, are the wrong sort of thing to be produced by a physical mechanism. Thus, it seems that the project of understanding phenomenal consciousness from a materialistic perspective is doomed from the start.

But it gets worse. For even if we could provide an account about how in fact phenomenal states are generated in humans, we still wouldn't know why this is so. We may ultimately be able to tell a detailed story of all the physical factors involved in the tokening of all of our experiences, but still be at a loss as to why these intricate physical happenings do result in those experiences. In other words, we may ultimately be able to specify a mechanism consisting of brain states and their interactions the functioning of which in fact always gives rise to phenomenal states, but which doesn't necessitate the presence of those phenomenal states. This is the point of the widely discussed zombie argument, which goes something like this. Consider an entity, named Z, which is an exact physical replica of myself. As such a replica, its qualitatively identical brain states interact in qualitatively identical ways to how my brain states interact, which entails that whatever the neurological mechanism is the proper functioning of which gives rise to phenomenal states in me also is present and properly functioning in Z. However, it is easily conceivable that Z doesn't share my phenomenal states, from which it follows that it is possible that Z lacks those states.1..2 But that means that the proper functioning of the relevant neurological mechanism doesn't guarantee the presence of phenomenal consciousness, which in turn entails that knowing what mechanism is responsible in me for experience does not give me an understanding of why this is so.

In fact, this problem of why particular neurological mechanisms give rise to experiences is just another way of highlighting the problem connected with the how-question discussed earlier. For what we lack even in the case in which we have specified the actual mechanism that gives rise to consciousness is some sort of conceptual connection between that mechanism and the resulting phenomenal states. This lack of a conceptual connection is responsible for the conceivability of the absence of consciousness even when the relevant mechanisms are functioning properly. To see the point more clearly, compare this case with a case in which detailed knowledge of the proper functioning of some mechanism does provide understanding of a phenomenon. Consider the question of why many pure samples of organic compounds, i.e., those that consist only of two or three of O, C, and H, produce "white smoke" when burned completely under conditions that are right for condensation. We know that the combustion of such organic compounds is analyzed as the oxidation of the elements constituting the compound. If the thermal energy is high enough to break the bonds between the elements of, say, two molecules of C2H6 (ethane), they will react with seven molecules of O2 to produce four molecules of CO2 (carbondioxide) and six molecules of gaseous H2O (water vapor), which condense at the right temperature and become visible as white smoke. The description of this process can be refined by specifying the exact mechanisms underlying chemical bonding. What is important for us is the feeling of inevitability one experiences once apprised of all the physical facts and of the right analysis of combustion. It seems inevitable that there be white smoke visible under the right conditions, given that this is how combustion works. If one were to imagine the mechanism of combustion functioning in an environment which is favorable to condensation and in which such an organic compound is burned completely, one cannot, or so it seems, conceive of the absence of white smoke.

The difference between this case and the case of consciousness appears to be one of the absence or presence of certain conceptual connections. While there seems to be nothing in our concept of a sharp pain that is even remotely related to our concepts of neurological processes, our concept of combustion, by contrast, involves essentially the concept of chemical binding with oxygen, which makes the explanation of the white smoke in the foregoing example so compelling.

With respect to the alleged mystery of phenomenal consciousness, the overall picture that emerges looks like this. In order to gain a materialistically acceptable understanding of consciousness, we need to find a neurological mechanism the proper functioning of which guarantees the presence of consciousness. Accordingly, a materialist appears to be committed to two things: (i) Materialistically legitimate explanations are mechanical explanations; and (ii) successful explanations are those mechanical explanations which, by guaranteeing the presence of the explanandum whenever the explanatory mechanism is functioning properly, close any conceptual gap between explanans and explanandum. Those who argue against materialism on the basis of considerations about phenomenal consciousness hold that no mechanical explanation of consciousness which guarantees its presence is ever going to be forthcoming. In a diagram, this argumentative strategy can be summarized as the following "iconic modus tollens:"

In this diagram, 'MAT' stands for the doctrine of materialism, 'LE' names the set of explanations that are legitimate from a materialistic point of view, 'SE' refers to the set of successful explanations, and finally 'C', with or without question mark, refers to the phenomenon of consciousness. The first line of the thus diagrammed argument claims that materialism entails that consciousness, as one robust phenomenon in the world, can be successfully explained by a materialistically legitimate form of explanation. The second line represents the negation of this claim: When it comes to consciousness, there is no explanation that is both materialistically legitimate and successful. We don't know where the phenomenon of consciousness fits. Even if we can specify a mechanism that in fact gives rise to consciousness and thus can locate the explanation of consciousness in the set LE, we still don't really understand consciousness, for the proper functioning of the mechanism doesn't guarantee the presence of consciousness, which entails that this explanation is not in the set SE (cf. the discussion of Chalmer's claim above). On the other hand (although this issue has not received much attention in the literature), if we found a successful explanation of consciousness, it will not be in the set LE. Either way, we have to conclude, applying modus tollens to the first two lines, that materialism is false.

This suggests that LE and SE are to be specified in the following way. The only materialistically legitimate form of explanation is mechanical. Only those explanations that let us understand how a certain phenomenon comes about in terms of an underlying mechanism that leads to it are acceptable for a materialist.2..3 On the other hand, the only successful explanations are those where the obtaining of the state of affairs specified by the explanans entails the presence of the explanandum. This is clear from how the zombie argument is used. For if the mere possibility of zombies is to show that consciousness cannot be explained in terms of a mechanism that in fact gives rise to consciousness, we must assume that the problem for materialism lies in the fact that the actual consciousness-producing mechanisms do not logically entail the presence of consciousness.

Thus, we get the following picture. First, materialism is said to be committed to the view that only mechanical explanations are legitimate. Second, the criterion for successful explanations is very stringent, for only those explanations are regarded as successful the explanans of which logically entails the presence of the explanandum. This latter criterion cannot be satisfied by any mechanical explanation of consciousness, and since only mechanical explanations are legitimate from a materialistic perspective, materialism is bankrupt when it comes to phenomenal consciousness. We might as well throw it out. An intriguing argument indeed, especially for its simplicity; but it is also unsound, as I will argue next.

3. Materialism Liberated

If materialism is committed to mechanical explanations as the only legitimate form of explanation, and if reasonable standards of explanatory success indeed rule out any explanation as successful that falls short of guaranteeing the occurrence of its explanandum, and if the conceivability of zombies indeed entails their possibility, then I agree that materialism is false. In other words, if the conceivability of zombies entails their possibility, the set of mechanical explanations and the set of successful explanations do not intersect, and materialism, with its commitment to their intersection vis-a-vis the phenomenon of consciousness, should be abandoned. Fortunately, even granting the dubious principle that conceivability entails possibility, materialism can be saved, for it is neither true that only mechanistic explanations are materialistically acceptable, nor is it reasonable to require that successful explanations guarantee the occurrence of their explananda. Put differently, the set of legitimate explanations and the set of successful explanations have to be enlarged for independent reasons, leaving open, once again, the possibility of finding an explanation of consciousness that is both successful and materialistically legitimate.

Before I discuss my reasons for advocating such enlargements of both LE and SE, let me make a couple of remarks about what I am not going to argue for. First, I emphatically do not want to argue that there can't be mechanical explanations of consciousness that ought to be deemed successful. For one thing, I do not accept the principle that conceivability entails possibility, which, as far as I can tell, is essential for making the argument against the possibility of successful mechanical explanations of consciousness. Secondly, I do not want to argue that we should not look for such mechanical explanations. I too find them extremely appealing and certainly hope that we will one day be able to provide one for consciousness. I will say more on the appeal of mechanical explanations below.

The goal of the following arguments is simply to show that no convincing case has been made against materialism yet. The plausibility the generic argument might have derives solely from an impoverished and mistaken notion of what counts as a successful, materialistically legitimate explanation. To show this, I will first discuss a well-known event in the history of science and suggest that this event constituted the beginning of the break with the idea that all and only mechanical explanations are materialistically acceptable. After that, I will call into question the reasonableness of the requirement that successful explanations guarantee the occurrence of their explananda by discussing a large class of exceptions to that requirement.

3.1 From Animism to Mechanism to Unification

The following brief detour through the history of science is intended to provide the background in light of which, I believe, the role of mechanical explanations can be better appreciated. This kind of explanation rose in importance in the 17th century, a period during which the so-called mechanical philosophy emerged as the scientifically respectable alternative to the then prevalent Renaissance Naturalism3..4. The latter was a view of the world at the center of which was the belief that all of nature is animated and contains principles of activity, which can be enlisted for the explanation of natural phenomena. The paradigmatic phenomenon for the Renaissance Naturalist was magnetism. In his treatise on magnetism, William Gilbert argued that the principles of sympathy and antipathy not only explained magnetic attraction and repulsion, but many other natural phenomena as well. The earth rotates, for example, because while it revolves around the sun, it perceives the magnetic field of the sun and reasons that, if it didn't rotate, one of its hemispheres would be burned, while the other one would freeze. Although still around after the Copernican Revolution - De Magnete was published in 1600 and reflects the new astronomy - the popularity of Renaissance Naturalism diminished to the degree to which Galileo's conception of inertial motion won adherents. The paradigmatic type of change, viz. change of place with uniform speed, no longer required an explanation in terms of active principles. Nature, really for the first time, could be conceptualized as entirely devoid of spirits and other occult entities and forces. In the wake of the Galilean revolution, nature came to be seen as consisting of particles in various shapes, which move around inertially along various paths, and whose interactions bring about all of the changes we want to explain.4..5

This programme was by no means confined to physics. Descartes, maybe the most ardent defender and developer of the mechanical philosophy, attempted to apply mechanical explanations to anything from vision to fetal development. For example, he developed what could be called the brewery model of fetal development, which he saw as the result of a fermentation process due to the mixing of the male and female seminal fluids. This fermentation produces heat, which "causes a number of particles to gather together in some part of the space containing them, and then makes them expand and press upon other surrounding particles. This is how the heart begins to form." (Descartes, 1664: 322)

The methodological maxim of the mechanical philosophy can be rather succinctly summarized in two claims. One, since there are no occult forces that act between particles instantaneously at a distance, such as the sympathies and antipathies of Renaissance Naturalism, no such forces are admissible for explanatory purposes. Two, every natural phenomenon has to be explained in terms of mechanisms, which are assemblies of particles of various shapes that interact with each other by contact only.

This extreme paucity in explanatory methodology could not be retained for long. There were mainly two reasons for the substantial modification, if not outright downfall, of the mechanical philosophy towards the end of the seventeenth and into the eighteenth centuries. First, Descartes and his fellow mechanical philosophers5..6 were never able to produce quantitatively accurate theories, and perhaps never even saw the importance of doing so (more on this below). The explanations of mechanical philosophers were usually only qualitatively appealing, but lacked any quantitative precision. For example, Descartes' account of free fall in terms of the interaction of vortices in the plenum that surrounds the earth was absolutely incapable of explaining the exact rate of uniform acceleration of freely falling bodies. Secondly, there was a proliferation of ad hoc assumptions about the particles involved in various interactions, especially when it came to chemical reactions. To exaggerate a bit, the explanation of just about every kind of chemical reaction was in need of its own kind of underlying mechanism. In other words, although some mechanism was imaginable for every phenomenon that one wanted to explain, it became increasingly clear that those mechanisms had nothing in common. The mechanical philosophy was methodologically and metaphysically austere, but it came at the price of losing any systematic unity.

To make a long story short, this very loss or lack of systematic unity seems to have been one of the factors that prompted Newton to abandon the basic tenets of the mechanical philosophy, a programme which provided the basis and background of his own early work (see Westfall, 1977: 139ff). The explanatory toolbox needed to be enlarged, not only to re-gain explanatory unity, but also to gain quantitative accuracy. With the introduction of the law of universal gravitation, Newton quite self-consciously admitted an all pervasive force acting instantaneously between particles at a distance. This of course looked dangerously similar to the sympathies and antipathies of the earlier Renaissance Naturalism. Newton was saved from the charge of re-introducing occultism by two facts. First, the postulated force was not some vague active principle like sympathy, but a quantity that could be treated in a mathematical model which gave (sufficiently) accurate predictions. Secondly, there was a slowly growing awareness that there will always be some things we have to accept as brute facts. The only trade-off Newton faced was between the mechanical philosophers' (very numerous) brute facts of the shapes of various particles and his own brute fact of the existence of some force that acts at a distance. This trade-off favored Newton, as his was the more unified theory in the sense that fewer facts had to be accepted as brute. The mechanical philosophers had to accept legions of facts about particles, while Newton's theory made do with universal gravitation (plus, of course, his three laws of motion, which, however, didn't look occultist). The development of Western science between the Renaissance and Newton can be characterized as, among other things, a shift in explanatory styles, from animistic to mechanistic to unifying models of explanation.

This characterization, although succinct, is a bit misleading. It suggests that there are no mechanical explanations after Newton, in the same way in which after Descartes, animistic explanations were absolutely inconceivable. This is not quite true. In fact, there is a clear sense in which Newton's explanation of, say, the ideal pendulum law, is mechanical.6..7 He postulates a world of particles that interact with each other in accordance with certain laws, from which it follows that certain regularities, such as the ideal pendulum law, are to be expected. The only difference to the classical mechanical philosophy of Descartes is that Newton allows for forces acting at a distance. Nonetheless, his explanations of Galileo's Laws, Kepler's Laws, and the like are in terms of an underlying mechanism, one that allows action at a distance, of which the other regularities are seen as special instantiations.

In what sense, then, is it justified to characterize the 17th century as a transitional period from mechanical to unifying explanations? I believe the following to be on the right track, although a full treatment would require a paper on its own. Towards the end of the 17th century, two things had become clear. First, the success of Kepler's new astronomy and Galileo's new physics had put an end to animistic explanation once and for all. In its place, mechanistic explanation had risen to the preferred, and really the only acceptable, style of explanation. But secondly, it had also become clear that mechanistic explanations are a dime a dozen. Some constraints had to be put on the imagination of professional explainers. For Newton, and probably others, such a constraint was found in the requirement of systematic unity. Mechanistic explanations had to obey the constraint that the mechanisms postulated form some sort of unified system. In Newton's mature work, this constraint of unity had begun to trump the central constraint of the classical mechanical philosophy, namely, the injunction against occult forces, i.e., forces that act at a distance. Given the choice between strict adherence to the injunction against occult forces on the one hand and the promise of a greater degree of systematic unity on the other, Newton went with the latter. It is in this sense that with him, we see Cartesian mechanisms sacrificed for the sake of unification. Thus we can speak of the shift from a mechanical to a unificationist model of explanation: When push comes to shove, metaphysical scruples about occult forces are less important than explanatory scruples about lack of unity and ad hocness.

But even if largely correct, doesn't all of this just show that unifying power had become an important constraint on acceptable mechanical explanations? If this question is understood as asking whether it isn't true that animistic explanations remained unacceptable after Newton, the answer is surely yes. However, if the question asks whether all post-Newtonian explanations were mechanical in the strict sense of the mechanical philosophy, the answer is no. To see this from a different perspective, consider the two different contrasts one may have in mind when claiming that some explanation is mechanical. On the one hand, such a claim might simply mean that the relevant explanation is not animistic in that it doesn't postulate any active, spirit-like principles in nature. In that sense, almost all explanations proposed since Newton have been mechanical. On the other hand, claiming that an explanation is mechanical might also entail that it specifies a system of parts that interact by contact only, at least when analyzed at the right level. Machamer et al. (2000) have recently argued that many biological explanations are of this sort. They specify systems of agents that react either in chemically or physically specifiable ways, but do not involve action at a distance. Such biological explanations are of the classical mechanical sort, at least until one looks for further explanations of the processes of chemical bonding, for example. At this deeper level, we find of course talk of weak and strong forces and the like. However, to be satisfying explanations, this level of detail doesn't seem to be required.

Now, in the same way in which explanatory success can be achieved by ignoring complications, such as the introduction of forces other than mechanical, that would arise, if one were to try to give the "full story" (which in many biological explanations we apparently don't need), explanatory success can also be achieved in the absence of specifying a mechanism responsible for the occurrence of certain phenomena. This, I believe, is one of the important lessons the scientific community learned from Newton's success. Once it had been demonstrated that the explanatory goal of systematic unity can trump the goal of specifying classical mechanisms, i.e. systems of particles that interact by contact only, it became conceivable that systematic unity may be sufficient for explanatory success. The first episode in the history of science that clearly illustrates the emerging awareness of the possibility of explaining a phenomenon without specifying a mechanisms is the development of Maxwell's electrodynamics.7..8

Maxwell clearly distinguished between two different roles mechanisms can play. On the one hand, they might be used purely for illustrative purposes, in an effort to aid the understanding of a phenomenon. This is the role he assigns to his model involving an ideal liquid flowing through a system of pipes, sinks, and sources in his paper "On Faraday's Lines of Force" (1856). On the other hand, Maxwell also tried, in his paper "On Physical Lines of Force" (1861-
62), to develop a vortex aether model that supposedly represented the true mechanics underlying electromagnetic phenomena. Although it was in this paper that Maxwell first derived his theory of light (as an electromagnetic phenomenon), he later abandoned his aether model due to the desire to 'clear the electromagnetic theory of light from any unwarrantable assumptions' (Maxwell, quoted in Morrison 2000: 81). In his "A Dynamical Theory" (1865), he laid out a theory that invoked experimental facts and abstract Lagrangian dynamics only. There was no more mentioning of molecular vortices. Indeed, Maxwell held the view that having described some phenomenon 'as a change in the configuration and motion of a material system, "the dynamical explanation of that phenomenon is said to be complete" (1965, vol. 2, p.418)'. (Morrison 2000: 100). This suggests that Maxwell accepted accounts as genuinely explanatory even if they do not specify the mechanisms underlying the dynamical behaviour of a system.

In Maxwell's case, the reason for this attitude seems to have been an almost verificationist worry about unobservable entities. However, this is not the only reason one may have for abandoning the requirement that good explanations have to specify mechanisms. As I argued earlier, the conceivability of explanatory success in the absence of explanatory mechanisms seems to be rooted in Newton's successful unification of terrestrial and celestial phenomena and regularities. In contrast to Morrison, I want to maintain that there is at least one sense in which unification might be a genuine explanatory virtue. If we assume that explanation ideally leads to understanding, and if we further assume that having to accept some phenomenon P as a brute fact is a paradigm case of not understanding P, we can cautiously infer that at least prima facie, reducing the number of phenomena we have to accept as brute increases our understanding of the (physical) world, as the number of things that are not understood has been reduced. This view of the relation between unification and explanatory power, now connected through the concept of understanding, allows us to make sense of the fact that Newton's new mechanics wasn't widely rejected despite the fact that it introduced entities dangerously looking like occult forces.8..9 It also allows us to make sense of Maxwell's claim that a mathematical description can be a complete explanation of a phenomenon even if there are no underlying mechanisms referred to.

The preceding discussion strongly suggests that it would become extremely difficult to understand major episodes in the history of science, if we were to believe that only mechanical explanations are materialistically legitimate. Both Newtonian Mechanics and Maxwell's electromagnetic theory of light are firmly footed in a general materialistic framework, but their success cannot be accounted for if we equate materialism with the classical mechanical philosophy. In both cases discussed, a further explanatory virtue did emerge, namely, the virtue of unifying power. More precisely, it turned out that unifying power cannot only trump the requirements of the mechanical philosophy, as was the case in Newton's decision to introduce an "occult" force, but may even replace whatever explanatory power mechanical explanations are supposed to have, should such explanations be unavailable.

More recently, we have witnessed other ways in which the role of mechanical explanations, viz. to give us a deep understanding of some phenomenon, can be carried out by explanations that are neither mechanical nor obviously unificationist. The explanation of planetary orbits provided by the General Theory of Relativity is in terms of the geometrical features of space-time and certainly does not specify any underlying mechanisms. Maybe more strikingly, Einstein's account for the Lorentz-contraction is clearly non-mechanical. But once apprised of all the relevant facts concerning moving clocks and rods and the measurement procedures involving them, one does have a deep understanding of Lorentz-contraction and time dilation.9..10 It is not clear to me how this sort of explanatory success should best be characterized. It's not clear how the understanding of the Lorentz-contraction is related to the unifying power of the special theory of relativity. But it is abundantly clear that the explanation is non-mechanical, especially when compared with Lorentz's own attempt to explain the contraction by referring to forces the moving body experiences. It serves, then, as another example of a materialistically acceptable non-mechanical explanation. Thus, it serves to underscore again that requiring from an explanation of some natural phenomenon that it be mechanical is overly restrictive. The set of materialistically legitimate explanations is larger than what those who argue for MINC would have us believe.

3.2 Statistical Explanations

According to my reconstruction of the generic argument for MINC and against materialism, an important premise is constituted by the claim that only those explanations whose explananda guarantee the presence of the explanandum are successful. And, as you will recall, since mechanical explanations don't deliver such guarantees, they can in principle not be successful. The task of this section is to show that this standard for explanatory success is far too stringent.

My argument is fairly simple. It consists in reminding the reader of the ubiquity of statistical explanations, in the case of which it is not merely logically possible that the explanandum is absent, even if all the explanans conditions are present, but instead probable to a higher or lower degree. Such statistical explanations replace strict and exceptionless laws with statistical laws. Inherent in this form of explanation is the real probability that the explanandum does not occur, even if all explanatory conditions are present. Moreover, it need not even be the case that the explanans bestows a high probability on the explanandum. The most well-known example of an explanation which confers only a low degree of probability onto the presence of its explanandum involves an affliction known as paresis, which is a sort of paralysis that results in 20% of patients with untreated latent syphilis. Consider Jones, who has developed paresis. Asked for an explanation, a physician will most like refer to the fact that Jones had left his latent syphilis untreated, and 20% of such people develop paresis. Thus, in Jones case, it is not only conceivable and therefore allegedly possible that he doesn't develop paresis despite the fact that he has syphilis, it is in fact overwhelmingly probable. Nevertheless, the explanation is successful in the sense that it lets us understand why Jones developed paresis.

A likely objection to my argument may take the form of the claim that I do not really understand why Jones developed paresis on the basis of the physician's explanation, for I do not understand what distinguishes those 20% of paresis-afflicted patients from other patients with untreated latent syphilis. Full understanding not only requires a knowledge of a necessary causal precondition of paresis (namely, the presence of syphilis), but also of the sufficient causal precondition (the hidden factors that distinguish Jones from other syphilitic patients). Comparing my example with the zombie argument, the objection looks like this. In the case of me, with all my phenomenal states, and my zombie twin, who (which?) doesn't have any of them, it is stipulated that we are apprised of all the relevant physical facts. Since even under this condition, we can still conceive of the zombie, we should conclude that we don't understand what consciousness is. In contrast, if we knew all the relevant physical facts about Jones and his illness, including the hidden factors that distinguish him from other syphilitic patients, we would fully understand the onset of his paresis, because the presence of all those factors would presumably guarantee that onset. If all the relevant factors are present, paresis necessarily develops. This may be right; nevertheless, the interesting fact is that we have some understanding of why Jones has paresis, even if we may not have full understanding.

What this discussion suggests, I believe, is that the defenders of MINC tacitly assume that only explanations that lead to a full understanding are successful explanations. In other words, understanding a phenomenon is an all-or-nothing affair. Either one fully understands a phenomenon on the basis of some explanation in the sense that the explanation guarantees the presence of the phenomenon, or one has no understanding whatsoever and has to accept the relevant phenomenon as a brute fact. This is why we can understand paresis but not consciousness. If apprised of all the relevant physical facts, one has a full understanding of paresis, which one doesn't have of consciousness, even if here, too, one knows all the relevant physical facts.

It seems unreasonable to not allow for degrees of understanding that fall short of full understanding to count as genuine understanding. By such a stringent standard, not much in the physical world is understood, maybe even nothing, for many of the explanations of real world phenomena are statistical in nature and thus lead at best to partial understanding. Developing an account of partial understanding falls well beyond the scope of the present paper. So, a short list of some of the areas where one is likely to encounter statistical explanations has to suffice for making my point. Those areas include, among others, economics, psychology, population genetics, statistical and quantum mechanics, medicine, and of course the automotive sciences. This pervasiveness of statistical explanations suggests that we make room for a notion of partial understanding, for otherwise, the very intelligibility of research projects carried out in the areas mentioned becomes highly dubious. The more general point is that we may be well advised to abandon theoretical physics as the paradigmatic model for what counts as a successful explanation, and the consequent genuine understanding, of some phenomenon. The bar set by that model is too high to be cleared by anything but highly idealized physical models, which in any case can only be applied to a small handful of real life situations under extreme contortions of abstraction and simplification (compare Cartwright 1999).

I have argued in this and the preceding section that a closer look at scientific practice suggests that the sort of explanations employed and the standards for explanatory success are decidedly different from what the defenders of MINC need them to be to make their argument successful. If we accepted their assumptions about what are legitimate and about what are successful explanations, most of the history of science would stop making sense, as would most research projects in fields other than physics (and maybe even many projects in physics). Unless we decide to be revisionist vis-a-vis scientific practice, the considerations adduced above provide good independent reasons for enlarging both the set of materialistically legitimate and that of successful explanations. The conclusion to be drawn from this observation is very modest indeed: No compelling reasons have been given as of yet for abandoning materialism, because the proposed broadening of the notion of successful, materialistically acceptable explanations opens the possibility for a successful materialistic explanation of phenomenal consciousness. In the concluding section, I will provide some brief remarks on the prospects for finding such an explanation.10..11

4. Understanding the nature of X

Here is an objection to the relevance, for the problem of consciousness, of my claim that statistical explanations provide at least partial understanding and are to that extent to be counted as successful and genuine. Remember how Jones's paresis was explained: It was explained as the result of untreated latent syphilis, which leads to paresis in 20% of the cases. The new objection to this counterexample holds that this explanation of paresis explains the wrong thing about it. Although it might provide a partial understanding of why Jones developed paresis, it does not illuminate the nature of paresis. Instead of telling me what paresis is, it just tells me (incompletely) why Jones has it.11..12

Compare the following explanation of Smith's consciousness. Suppose Smith has lost, due to some brain trauma, the ability to experience colour. Suppose further that recent advances in neurology have made it possible to surgically intervene in such cases with a success rate of about 20%. Simply imagine a colour chip that, if integrated into the appropriate neural circuitry, results in one out of five patients with the relevant brain trauma to report a restoration of their colour experiences. When asked why Smith has colour consciousness again, the surgeon would refer to the fact that the colour chip has been implanted and that in 20% of the cases, this procedure is successful. Of course, this explanation of why Smith regained his colour consciousness does not do anything to explain what colour consciousness is.

The parallel between the explanations of Jones's paresis and Smith's colour consciousness is obvious. Thus, all that my original example showed is that there are cases in which the emergence of a particular condition or phenomenon can sometimes be statistically explained. But, so the objection continues, this is not what we should be worried about as materialists. What we should be worried about is the question whether we can explain what consciousness is. At stake is the nature of consciousness, and when the nature of some X is the explanatory target, statistical explanations are simply not good enough. One simply doesn't understand the nature of some phenomenon if all one can do is point to some conditions under which it always, usually, or at least sometimes, occurs.

Point taken. If the explanatory target is the nature of some phenomenon, statistical explanations of its occurrence won't do. The deeper question that seems to be lurking here, however, is why the natures of some phenomena appear like reasonable explanatory targets. What are the reasons behind the demand that for certain phenomena, we need to know the essential conditions of their presence in order to be said to understand them? Moreover, which ones are the phenomena for which it is reasonable to demand an account of their nature? What distinguishes these phenomena from those others, with respect to which we are quite satisfied if we have an account of their behaviour and the conditions of their occurrence?

These I believe are questions that haven't received sufficient attention as of yet. The defenders of MINC presume without argument that phenomenal consciousness is a phenomenon for which knowledge of its nature can reasonably be demanded. This might be so because they believe that such a demand makes sense for every phenomenon. But that is not the case. Take, for example, the phenomenon of motion. During the Middle Ages, there was a heated debate about the nature of motion. Some argued that motion is simply a change in the spatial relation between one object and other objects, while others held that motion is a genuine property of objects, on a par with colour and temperature (see Lindberg 1992: 292f.).12..13 The debate remained unresolved, and it eventually disappeared by simply becoming irrelevant with the emergence of Galileo's new dynamics. In his approach, the question of the nature of motion was dropped in favor of questions about the regularities characteristic of systems of moving entities. From our modern perspective, it is hard to even imagine what an acceptable answer to a question about the nature of motion would look like beyond the specification of the relevant regularities. To be sure, these include the laws of mechanics as the investigation into the causes of changes in motion. However, the role mechanics plays here is not one of specifying hidden mechanisms the proper functioning of which would somehow guarantee the presence of motion. Motion is simply the wrong sort of phenomenon to allow for such a reductive, mechanistic explanation.

Of course, there are phenomena the nature of which we have successfully explained in terms of underlying mechanisms. Chemical bonding is one such phenomenon, genetic inheritance is another one. This raises the question as to which phenomena are and which ones are not susceptible to an explanation of their nature. I have no idea how to answer this question in general. One answer, however, that is clearly false consists in the claim that it is physical phenomena the nature of which can be understood, while it is the nature of non-physical phenomena that cannot be understood in the same sense. The example of motion shows that this answer is the wrong one.

I believe that these considerations make it reasonable to put on hold questions about the nature of consciousness, at least until it can be shown that the phenomenon of consciousness satisfies the criteria for a phenomenon the nature of which it is reasonable to investigate, whatever those criteria will eventually turn out to be. At the very least, in the absence of a convincing argument (instead of reliance on pre-theoretical intuitions) that consciousness is more like chemical bonding than it is like motion, the alleged inexplicability of its nature due to the impossibility of specifying consciousness-guaranteeing mechanisms is no reason for abandoning materialism. After all, the parallel inexplicability of the nature of motion is no reason for abandoning materialism either.

On the positive side, the possibility that consciousness is more like motion than it is like chemical bonding may contribute to a more appreciative attitude towards possible regularities we might find in the occurrence of particular phenomenal states and their relations to neurological states and/or processes. Correlations that can be established between occurrences of phenomenal states and certain processes in the brain, while certainly not explanatory in themselves, may be less readily dismissed than they are now in the more philosophical literature on consciousness. Of course it is correct to point out that such correlations don't explain consciousness. But finding such correlations is only the first step in a science of consciousness. The real task is to explain why these correlations hold. Here, it is important to remind ourselves that correlations can be explained in at least three ways, only two of which seem to have come to the mind of defenders of MINC (possibly also due to their impoverished notion of explanation). We can explain correlations through causal relations, by either identifying one of the correlata as the cause of the other one, or by identifying them both as effects of an underlying common cause. But we can also explain correlations by embedding them in a more fundamental and unifying framework. This is how Newton explained the correlations expressed in Galileo's and Kepler's laws. The hope, then, is that we may find some fundamental and unifying neurological story from the perspective of which the observed correlations between certain neurological states and phenomenal states don't come as a surprise, at least not as a big one. Of course, whether any such theory is forthcoming is largely an empirical question. But it should certainly be avoided to dismiss the search for correlations by insisting that the won't do anything to illuminate the mystery of the nature of consciousness. Maybe there simply is no intelligible question about the nature of consciousness.13..14

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