4.4 Physicalism and the hard problem

I introduced the hard problem as an explanatory problem – the problem of explaining how consciousness arises. But it can also be presented as a metaphysical problem – the problem of saying what kind of phenomenon consciousness is, and, more specifically, whether it is a physical one. In this section I shall say something about this aspect of the hard problem and its relation to the explanatory one.

The terms ‘physical’ and ‘physicalism’ (the view that everything is physical) are used in a number of different senses and it is easy to become confused by them. Some writers who count as physicalists in one sense of the term count as anti-physicalists in another. (The same goes for ‘materialism’, which is sometimes used interchangeably with ‘physicalism’.) I shall distinguish some important senses and give them labels, but you should note that other writers draw the distinctions in different ways and use the terminology differently.

In one form, physicalism is the view that everything in the universe is composed wholly of the basic entities and forces postulated by modern physics (electrons, protons, gravity, electromagnetism and so on). It is the view that, as John Haugeland puts it, ‘if you took away all the atoms, nothing would be left behind’ (Haugeland, 1982, 96). I shall refer to this view as substance physicalism. (Substance dualism, on the other hand, is the view that the universe also contains other entities and forces in addition to the basic physical ones – immaterial souls or psychic energy, for example.) Now I am going to assume that substance physicalism is true. This reflects the prevailing view among contemporary philosophers of mind, including some who would describe themselves as being, in another sense, non-physicalists. The modern debate over physicalism focuses on other claims, not about substances, but about properties. (If you are sympathetic to substance dualism, you should not conclude that the rest of this course will be irrelevant to you; I shall explain why shortly.)

Suppose substance physicalism is true. Still, questions remain about the properties of things. Let me begin by introducing the notion of a basic physical property. By this I mean a property invoked by physicists, such as mass or electrical charge, or a property that can be defined in terms of the properties invoked by physicists, such as that of being composed of atoms of a certain kind. Now if physics describes the basic components of the world, then there is a sense in which the basic physical properties of things are the fundamental ones. But, of course, things possess many other properties in addition to their basic physical ones. Take me. I have various basic physical properties. For example, I have a certain mass, I am composed of millions of molecules arranged in elaborate structures, I am the site of numerous complex electrochemical processes. But I also have many other properties: I am alive, I am human, I have a digestive system, I belong to blood group O, I like Bob Dylan, I currently have a slight headache and so on. Let us call these high-level properties. But how are these high-level properties related to my basic physical ones? The question could also be phrased in terms of facts. For each of my properties there is a corresponding fact – the fact that I possess the property. So another way of putting the question would be to ask how the high-level facts about me are related to the basic physical facts about me. Most modern debates about physicalism are about the answers to questions like these.

Property physicalism is the view that high-level properties are not fundamentally distinct from basic physical ones. They are not new features of the world, in addition to the basic physical ones, but just those same features under different guises. So, for example, my having a digestive system is not an extra property of mine, over and above the basic physical ones. Rather, it consists in my having certain basic physical properties – having certain basic physical components arranged in a certain way and performing certain functions. Similarly for all other high-level properties. Or, putting it in terms of facts, high-level facts are not extra facts, over and above the basic physical ones; rather, they are just redescriptions of the basic physical ones. There is, it is true, a sense in which high-level facts plainly are different from basic physical ones: the claim that I have a digestive system does not mean the same as the claim I have certain basic physical properties. But – the physicalist will say – there is just one underlying state of affairs which makes both claims true. This is sometimes expressed by saying that once God fixed the basic physical facts, he fixed all the facts; there was no more work for him to do (Kripke, 1980). (The reference to God need not be taken literally – it is just a vivid way of making the point about the relation between the different properties.)

Property physicalism contrasts with property dualism. This is the view that some high-level properties are fundamentally distinct from basic physical ones – that they are additional features of the world, over and above the basic physical ones. Or, putting it in terms of facts, some high-level facts are extra facts, distinct from the basic physical ones. So when God fixed the basic physical facts, he still had more work to do: he still had some high-level facts to fix. If he had pleased, God could have created a world which was exactly like ours in all its basic physical details but which didn't have the same high-level properties. It is worth stressing that property dualists do not claim that all high-level properties are distinct from basic physical ones, but only that some are. Thus, while property physicalism is a general claim about all high-level properties, property dualism comes in different versions, each concerned with a different high-level property or group of high-level properties.

Of course, the dualist about life may accept that there is a regular correlation between the presence of certain basic physical properties and the presence of life. After all, we don't find life in just any old physical structure but only in certain highly organised ones. The dualist may even say that it is a law of nature that when certain basic physical properties are present, then life is, too. But, they will say, it is a contingent fact that this law holds and it could have been different. (Compare the way that light has a certain speed in our universe but could have had a different one.) The physicalist, on the other hand, will deny that it is a contingent fact that certain basic physical properties are correlated with life. Rather, they will say, there is nothing more to being alive than having the right set of basic physical properties, and the latter could not occur without life.

There is one more distinction to make before we move on. Property physicalism is the view that high-level properties are not fundamentally distinct from basic physical ones, but there are stronger and weaker versions of this view. According to the stronger version, high-level properties reduce to basic physical ones. That is, each high-level property can be identified with a single basic physical property (or single set of such properties) in all its instances. This is sometimes expressed by saying that the two properties are type-identical. This strong form of property physicalism is not plausible. Some high-level properties reduce to basic physical ones; for example, having blood group O reduces to having blood of a certain molecular composition. But, as I mentioned earlier, most high-level properties can be constituted in more than one way. For example, the digestive system involves quite different structures in different animals. The same goes for mental properties, too. An alien might suffer from headaches and like Bob Dylan despite having a completely different brain chemistry from me. It does not follow, however, that these properties are fundamentally distinct from basic physical ones; it remains open that they are realised in basic physical properties. That is, every instance of these properties might be identical with an instance of some basic physical property – the nature of the latter varying from case to case. So a weaker and more plausible version of property physicalism holds that high-level properties either reduce to or are realised in basic physical ones.

This weak form of property physicalism is the most popular contemporary version of physicalism and the chances are that when you come across the word ‘physicalism’ (or ‘materialism’) in a contemporary book or article it is this weak form of property physicalism, or something very close to it, that is meant (though there will be exceptions and you should always check to make sure). For convenience, I too shall use the term ‘physicalism’, without qualification, to refer to this position. I shall also assume that physicalists in this sense endorse substance physicalism, though I shall not be discussing this aspect of their position. Two more terminological points. I shall use the term ‘physical’ in a broad sense to refer both to basic physical properties and also to high-level properties that are realised in basic physical ones. Likewise, I shall use the term ‘physical facts’ both for facts about basic physical properties and also for facts about high-level properties that are realised in basic physical ones. Thus, the physicalist, in the sense just defined, holds that all properties are physical properties and that all facts are physical facts.

Figure 4 Varieties of dualism and physicalism

It is worth stressing that most writers accept that many phenomena are physical in the broad sense defined above. It is widely accepted, for example, that chemical, biological, neurological and functional properties are physical ones, and in the rest of this book I shall assume that this is the case. The question is whether consciousness is a further, non-physical property, over and above these.

I now want to link up this discussion of physicalism with the earlier discussion of reductive explanation. The crucial thing to note is that strong naturalists are committed to physicalism, in the sense just defined. For they hold that all high-level properties can ultimately be explained in basic physical terms. And such explanations will be possible only if high-level properties are not fundamentally distinct from basic physical ones. Recall how we explained the television's power to display moving images. The explanation worked because it was clear that the properties mentioned in the explanation were sufficient for the existence of the power. With those components, working in that way, the set simply could not fail to display moving pictures. There was nothing more to it. In short, the properties cited explained the power because they realised it.

For the same reason, if a high-level property is distinct from basic physical ones, then it will not be possible to explain it in basic physical terms. Suppose property dualism about life were true – that life was an extra property, over and above basic physical ones. Then it would not be possible to give a reductive explanation of life in basic physical terms. For even if we were to identify all the various chemical and physical processes associated with life, there would still remain a mystery: Why does life emerge when those properties are present? If life were something extra, over and above those properties, then identifying them would not fully explain its existence. (I should add that very few people are property dualists about life; as I mentioned earlier, it is widely accepted that organic processes can be reductively explained in terms of chemical and physical ones.)

To sum up: if you believe that all high-level phenomena can be reductively explained in basic physical terms, then you are committed to the view that high-level properties are not fundamentally distinct from basic physical ones – that is, you are committed to physicalism.

The converse also applies. If consciousness is a physical phenomenon, then it should be possible to reductively explain it by identifying the physical properties in which it is realised. When these properties are described in detail, it should be obvious that they are sufficient for the existence of consciousness – just as it was obvious that the various electrical and mechanical components of the television were sufficient for it to have the power to display images of distant events. The question of whether consciousness is a physical phenomenon and that of whether it can be reductively explained are thus two sides of the same coin: considerations for and against the one claim count equally for and against the other.

I shall close this section by addressing some worries that may have been raised by the preceding discussion. First, I mentioned that I was going to assume that substance physicalism was true. The question we shall be considering is whether the property of consciousness is a physical one, not whether the substance which possesses this property is. I shall assume that this substance is simply the brain. But what if you do not share this assumption? What if you believe that we have a non-physical soul as well as a brain? Won't you find the debate about properties irrelevant? I don't think you should. For it is quite possible to hold both that we have a non-physical soul and that consciousness is a property of the brain. Some mental properties may be physical, even if others are not. (And, as I noted earlier, it is hard to deny that consciousness is at least very closely related to the brain; we know that chemical changes in the brain can affect consciousness and that damage to the brain can remove it.) If, on the other hand, one of your reasons for believing in substance dualism is that you think that consciousness is not a physical phenomenon, then you will need to consider the arguments for the view that it is, and think about how you would respond to them. You might also want to consider whether you really need to endorse substance dualism, as opposed to the weaker property-based version.

Secondly, you may feel uncomfortable with the emphasis I have placed on a reductive, physicalist explanation of consciousness. Why should we bother to look for such an explanation? Isn't the whole approach dehumanising?

Wouldn't it be better to turn to novelists and poets for an account of our inner lives, rather than to scientists? There is something in this. If we want descriptions of what consciousness is like – the subtle shades of perception, sensation, emotion and thought – then a reductive explanation is likely to leave us cold. This is true across the board. An account of how a phenomenon is constituted will not tell us much about how it affects us. If you want to know about the emotional impact of the Mona Lisa, then a chemical analysis of the paint will not satisfy you. But it would be a mistake to think that this is an objection to reductive explanation. High-level descriptions of a phenomenon are not made redundant by a reductive explanation of it. The two serve quite different purposes: one tells us how the phenomenon appears to us, the other how it is constituted. Indeed, the two approaches may be complementary; close observation of a phenomenon may provide hints as to how it is constituted, and learning how it is constituted may illuminate our observations of it. A chemical analysis of a painting may help us to see it in a new way – to see new relations between its colours and to understand why they have the effects they do. There is no need, then, to fear that a reductive explanation of consciousness will replace the descriptions of novelists and poets; indeed it may help to enrich them.

Thirdly, is physicalism really a coherent position? Some people object that it is not, since it is not clear what counts as a basic physical property. After all, physicists keep revising their catalogue of the basic physical entities and forces, and they will probably continue to do so for some time (Crane and Mellor, 1990). This is a fair point, but not, I think, a fatal one. Physicalists can respond that their position involves an open-ended commitment – a commitment to regard as fundamental the properties posited by our current physics, whatever these happen to be. (Compare how being a law-abiding citizen involves an open-ended commitment to abide by the laws in force at the time, whatever these may be.) This means that physicalism is not a tightly defined doctrine, but in practice this does not matter too much, especially so far as philosophy of mind is concerned. For what is chiefly at issue in debates about the mind is whether mental properties are identical with, or realised in, non-mental ones. The exact nature of these non-mental properties is a secondary issue. So physicalists can live with some vagueness in their position (Papineau, 1993, 2002). Of course, if physicists were to decide to include mental phenomena in their catalogue of the fundamental entities and forces, then it would be a different story. But in that case the whole debate would take on a completely new aspect and everyone would have to rethink their position.