The Physical World Author(s): Barry Stroud Source: Proceedings of the Aristotelian Society, New Series, Vol. 87 (1986-1987), pp. 263-277 Published by: Blackwell Publishing on behalf of The Aristotelian Society Stable URL: http://www.jstor.org/stable/4545066 Accessed: 24/01/2009 07:49 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showpublisher?publishercode=aristotelian. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact support@jstor.org. The Aristotelian Society and Blackwell Publishing are collaborating with JSTOR to digitize, preserve and extend access to Proceedings of the Aristotelian Society. http://www.jstor.org
XV*-THE PHYSICAL WORLD by Barry Stroud What sort of thing is the physical world? I do not ask for details about what the world is like that might be supplied by the physical sciences. I have in mind the appeal to the physical world that is made in philosophy, especially in the metaphysical thesis of materialism or physicalism: the world is purely physical, or the physical world is the only world there is, or the only world that is real. Traditionally, materialism was a thesis about objects-only bodies exist, the only things that exist are material things, the world contains nothing but physical entities. It was a monism of physical substances, as opposed to a dualism of bodies and minds, or an idealism of only minds. But what the world is like is not settled just by what sorts of things are in it. Given the very same collection of things, you could have quite different worlds, depending on how those things were related to each other and how they behaved. What is true of the things that exist counts just as much for what the world is like as what sorts of things there are. Why philosophers have traditionally gone in for classifying or counting the number of kinds of objects there are, as opposed to asking what is true, is a big and difficult question which still needs an answer. Perhaps part of the explanation is the thought that the sorts of things that exist put limits on what the world could be like by restricting what is true of the things that exist. If the 'essence' of body was extension, for example, and only bodies existed, then everything that was true of what exists had to be a 'mode' of extension, since bodies could have only those properties that are 'modes' of their essential property. What the world is like in general could be deduced from the essences of the things it contains. Whatever its source, ontology has been the attempt to answer the question 'What is there?' or 'What exists?' But that alone would not tell us what the world is like. That can be done only by saying what is true of the things that exist, not * Meeting of the Aristotelian Society held at 5/7 Tavistock Place, London WC1, on Monday, 8 June, 1987 at 6.00 p.m.
264 BARRY STROUD just by listing the things, or counting the different kinds they belong to. A physicalism that said only that every thing that exists is a physical thing would seem to exclude from the world only certain non-physical entities like Christian souls or Cartesian minds.' But what the world is like could be otherwise as rich as you please, as long as all the entities in it were physical. It could be a world of coloured things with taste and smell. A lemon is a physical thing, and it has colour, taste, and smell. And many physical things are hot or cold, and make a noise. The things in such a world could be beautiful or ugly, valuable or worthless, as far as a physicalism of entities was concerned, as long as they were all physical things. It could also be true that many things in the world think, feel, and act, that their actions are sometimes good, sometimes bad, and that some things are virtuous and others evil. That is all true of human beings, and they are all physical things. It could be true that many things in the world have meaning, some mean one thing, some mean another. In short, nothing would seem to have been excluded except some spooky things which probably could not have been made sense of on their own as entities anyway. Physicalism does not necessarily want to exclude all such facts from the world. But it at least involves the project of understanding some or all of them in a certain way. They will be acceptable as facts only if they can be fitted in to the purely physical world. But then 'the physical world' as it appears in that requirement cannot be understood as referring simply to a collection of physical things. Obviously what is at stake is the idea of a physical fact, not just physical entities. The physical world consists entirely of physical facts. What is not a physical fact is not part of the physical world. And physicalism is the thesis that the physical world is the only world there is, or the only world that is real. Philosophers, even physicalists, are divided on the rich variety of facts I have mentioned. Some say it is not a fact at all that some objects are coloured; others say it is a fact, and so must ' It would also exclude all abstract objects, and hence all abstract objects widely believed to be essential to the truths of mathematics. Here I simply leave to one side the question of how physicalism could accommodate mathematics.
THE PHYSICAL WORLD 265 be a physical fact, although they have difficulty specifying precisely what kind of physical fact it is. Many appear to hold that it is not a physical fact that things have taste or smell or make a noise; others would give a 'dispositional' physical account of what- that amounts to, similar to their treatment of colour. Few are tempted by a physical explanation of beauty, or worth, or virtue. There are held to be no such features in the world at all. But almost everyone seems agreed that thinking, feeling, and action occur, and physicalism has concentrated most on accommodating them. The widespread idea that thoughts, feelings, actions, and so on are somehow identical with various physical phenomena is generated precisely by the thought that the world is a purely physical world, and so if such phenomena are going to fit in anywhere they will have to be entirely physical. On the question of the meanings of things the consensus seems to go the other way. There has been little temptation to identify the fact that the word 'cat' means cat with any physical fact. In the purely physical world, there is simply no fact of the matter. My interest here is not in assessing the merits of these different physicalist accounts of colour, beauty, goodness, mind, action, or meaning. I am interested in what they all have in common -the idea of the world as purely physical, or the idea of a purely physical world. The uncompromising physicalist who would exclude certain phenomena from the physical world and the more accommodating physicalist who would include them by analyzing or identifying them in purely physical terms both share an idea of the physical world and its exhaustiveness. It is meant to be the only world there is. What is the physical world in that sense? How is that idea of an exclusively physical world arrived at? It is the idea of the totality of physical facts, but how are the purely physical facts to be identified? We cannot say simply that they are all and only those facts that are physical in the sense in which physical things are all and only those things that are physical. For that we would need a notion of what it is for a thing to be a physical thing. Do we any longer have an idea of the essence of the physical- those properties which any physical thing must have? Does physics even think at the most fundamental levels of particular things or entities at all? In the seventeenth century it seemed easier.
266 BARRY STROUD Extension, or extension and impenetrability, defined the physical. Even if we accepted that criterion of a physical thing it would not work for physical facts. Physical facts cannot be said to be those facts that are physical in the way that physical things are those things that are physical. Things that are physical are things that are extended and impenetrable. But facts are not extended and impenetrable at all. They have no spatial location or dimensions. Nor do they have any of the other features that might be thought to be definitive of physical things. Obviously it would not help to say that physical facts are any facts which involve or hold of physical things. We already saw that a mere physicalism of entities is too tolerant. It would not even exclude a thing's beauty from the physical world. Michelangelo's Piet'a is a physical thing, and it certainly is beautiful, so on this test it would be a physical fact that it is beautiful. To reply that beauty is not a physical property of physical things would be no help. Physical properties and relations are no more easily definable than we have found physical facts to be. Even assuming we have a definition of a physical thing, the same problem arises for physical properties and relations as arose for physical facts. To say of a property that it is a physical property is not to say that it is a property and it is physical in the sense of being extended and impenetrable, or anything else that might be thought definitive of physical things. Properties are perhaps instantiated at various points in space, but they do not literally have spatial dimensions, as physical things on this criterion do. Properties, even physical properties, are not in that sense things that are physical at all,2 so the same criterion of the physical will not work for both. It looks as if the only way to identify facts as of one type rather than another-and therefore in particular as physical-is in terms of the vocabulary or concepts in which they are expressed. Purely physical facts are facts stated in a purely physical vocabulary, or with purely physical concepts, just as economic facts are those stated in economic terms or concepts. The question then is: which are the purely physical concepts? What 2 Properties, being abstract, could not be accepted by a physicalism that says that every thing that exists is a physical thing. The need for abstract objects is a more serious difficulty than many physicalists seem to suppose. It should encourage the idea I am urging that what is crucial is a physicalism of facts, not of things.
THE PHYSICAL WORLD 267 is the physical vocabulary? If we can pick out the terms that express or define the domain of the physical, we could then identify the purely physical properties and relations as those denoted by the terms of that physical vocabulary. Physical facts would then be all those facts in which those properties or relations are truly ascribed to physical things. The physical world would then comprise all and only the physical facts. Physical terms, concepts, or properties might be identified as those employed or ascribed in physics or the physical sciences. This appeal to the sciences is no real advance unless we have some independent idea of the physical, some conception of what it is that makes a science a purely physical science. In the seventeenth century, when the essence of the physical was thought to be known, it could perhaps have been fixed in terms of subject-matter. It was thought to be known what any future physical science would have to deal with if it was dealing with the physical at all. I doubt that we have any comparable confidence today about the essence of the physical, or what any future physical science will have to be like. I believe that this is a more serious difficulty for physicalism than it is usually taken to be. There is no satisfactory way to identify the type of fact which for the physicalist is the only kind of fact there is. I will return briefly to this difficulty at the very end. But for now I set it aside and simply take as given the physical sciences as we now understand them. That still leaves the limits vague, but for my present interest in the notion of the physical world I don't think that matters. Can we say that the physical world is the world as described in the physical sciences? We pick out, or are given, the physical sciences, and then what all the truths expressible in those terms describe is what we think of as the physical world. That does give us a notion of a world of some kind or other, but I do not think it is enough to give us the physicalist idea of the physical world as a complete world, as the only world there is. Let us agree that the truths of the physical sciences contain no mention of the colours, tastes, smells, and sounds of things. They speak only of physical processes of reflection and transmission of light, of vibrations in the air, of cells being affected in certain ways, and so on. The physical sciences say nothing about the beauty or the economic value of anything either. They mention no
268 BARRY STROUD thoughts, feelings or human actions, and nothing about their goodness or badness.. And they ascribe no meaning to anything. They mention only purely physical relations and interactions among things in the physical world. If the physical world is the world as described in the physical sciences, then, there is a clear sense in which colour, beauty, thought, meaning, and so on are not part of the physical world. But all that means is that such phenomena are not mentioned in the physical sciences. It does not mean that there are no such phenomena in the world. It does not mean that it is not a fact that lemons are yellow, the Pieta is beautiful, human beings think and act, some of the things they do are bad, 'cat' means cat, and so on. The more accommodating physicalist who accepts some or all of these facts can do so only by showing how they are somehow identifiable with facts of the physical world after all. That means that they must be identifiable with facts expressible in purely physical terms if they are going to be facts at all. But why? What is the source of that demand? It is equally true, for example, that in the expression of facts about the interactions among economic agents expressed in purely economic terms there is no mention of the height and weight of the agents involved, or of the atoms or electrons they are made up of. But that does not lead us to conclude that nothing in the world has any height or weight, or that there are no atoms or electrons in the world. Nor does it lead us to think that if we do want to accept such facts about height and weight and atoms we must somehow identify them with facts expressible in purely economic terms. We acknowledge that such things simply do not get mentioned in economic descriptions, and that in no way impugns their status as facts. If we thought of the physical world as simply the world as described in the physical sciences we could draw no stronger conclusion about facts of colour, beauty, thought, meaning, and so on. The difference cannot be simply that we think the world is in fact a physical world. All that means, as far as I can see, is that we believe physical truths, we think there are physical truths, that statements expressed in purely physical terms are true. And if that is what it means to say that the world is a physical world we must grant that the world is an economic world too. We believe economic truths, or we think there are economic truths, that
THE PHYSICAL WORLD 269 statements expressed in purely economic terms are true. We believe that statements expressed in colour terms are true as well, and statements in psychological and evaluative and intentional terms. So in the sense in which we think the world is a physical world we also think it is an economic world, a coloured world, a world with thoughts and feelings in it, and good and bad things, meaningful things, and so on. We can accept all of that while agreeing that none of it gets mentioned in the physical sciences. They simply leave it open whether facts that are not stated in the terms of the physical sciences nevertheless hold in the world. If we think they do hold, their not being stated in the terms of the physical sciences give us no reason to look askance at them or to insist that they be shown somehow to be expressible in the terms of the physical sciences after all. The world as described in the physical sciences, then, isjust the world as described in one way rather than another. The physical sciences mention only certain aspects of the world, other sciences or other ways of describing the world mention others. We think of it in terms of certain aspects at one time, with certain interests and purposes, and in other terms at other times, with other interests. Whenever we think of something we think of only certain aspects of it, not all. That seems to be what makes it possible for us to think about anything. If we had to think everything that is true of a certain thing in order to think anything that is true of it, it would be impossible. We abstract certain features for consideration and ignore the rest. But we do not conclude from our thinking of something only as having the features F, G, and H that the thing has only the features F, G, and H and no others. Physicalism is the thought that the world is only physical; that the physical facts are the only facts there are, and that they make up the physical world. But the physical world as it appears in that thought cannot be just the world as described in the physical sciences. Can we say, then, not that the physical world is the world as described in the physical sciences, but that the physical world is the world the physical sciences describe? That might seem to free us from the earlier restriction to mere vocabulary or ways of describing the world and let us get through to the world itself that those descriptions are about. That seems to be what the
270 BARRY STROUD physicalist is trying to get at. But that obviously does not yield the physicalist conception of the physical world either. There is so far no reason for saying that the world the physical sciences describe is not the very same world as the world economics describes, or the world colour statements describe, or the world psychological or evaluative or intentional statements describe as well. That would seem to be just the consequence to draw from the reflection that there are many different aspects under which we can think about something, and our thinking of it in only one of those ways does not render the others false or inapplicable. There is the world, and it can be thought about and described in many ways. Each way singles out certain aspects, and there is no need to think of it in all possible true ways in order to think of it in one or two. Here the idea of the world seems to come to nothing more than the idea of everything that is the case. The world comprises all the facts, everything that is said to be so by whatever is true. It is therefore as rich as the rich variety of statements that are true. An idea of the physical world as the world the physical sciences describe would in itself give no reason for declaring something not expressed in the physical sciences to be no fact at all, or for requiring that it be somehow explicable in purely physical terms if it is going to be a fact of the physical world. Any statement that is true will already state a fact of the physical world if the physical world is thought of only as the world the physical sciences describe. In the world that the physical sciences describe it is true that many things are coloured, some things are beautiful, others think, feel, and mean things, and so on. At least the truth of the physical sciences gives us no reason to think anything to the contrary. I therefore think the idea of the physical world as the only world there is, or the idea that the only facts there are are physical facts, cannot be derived simply from consideration of the physical sciences alone and the observation that they describe only the physical aspects of things. One further thought is that the physical sciences are unique in being universal; they apply to everything there is in a way economics or psychology or even colour statements do not. That in itself, even if true, does not seem to me to support the idea that physical facts-those stated in the physical sciences-are the only facts there are. Either it amounts to the thesis that the only entities that exist are
THE PHYSICAL WORLD 271 physical things-which is compatible with there being a wide variety of facts that hold of those physical things-or it is the claim that many things with physical properties do not also have economic or psychological or colour properties at all. But that does not even begin to suggest that nothing has any economic or psychological or colour properties, that it is not a fact that people do stand in economic relations to one another, that they do think and feel and act, or that lemons are yellow. Nor does it give a reason for insisting that those are facts of the world only if they can somehow be identified with facts expressed in the physical sciences alone. But there is another idea of the universality of the physical sciences which has seemed to give them a special position. They do not just describe the world or state what is true. They can also be used to explain why things happen as they do, why the facts are as they are. The explanatory powers of the physical sciences have been thought to be universal in a way that might give content to the idea that the physical world is the only world there is. If appealing only to the truths of the physical sciences alone were enough to explain everything that happens or is the case, that might be thought good reason to believe nothing other than those physical truths. Everything else we took to be a fact would have to be fitted in somehow to that core physical story. Can we arrive at the idea that the physical world is the only world there is, that physical facts are the only facts there are, from the prospects of universal physical explanation? I think the answer is 'No'. It looks to me as if this proposed explanatory route could lead in the direction of the physicalist conception of the world only if we already had the idea that the world is exclusively physical or that the physical world is the only world there is. An explanation needs an explanandum-some fact to be explained. Here I speak only of explaining why something or other is so. Take the case of colour. Is there a purely physical explanation of why lemons are yellow? If there is, then it must be acknowledged as a fact of the world that some things are yellow and hence coloured. Otherwise there would be nothing for the physical explanation to explain. But some things' being coloured conflicts with the uncompromising physicalist view that nothing in the physical world is coloured and the physical
272 BARRY STROUD world is the only world there is. It does not conflict with the more accommodating physicalist view to the effect that a thing's being coloured consists in its being such as to cause human beings or other perceivers to perceive colour and to believe that things are coloured. On that view, physical explanation which makes no mention of colour is held to be sufficient to explain why human beings perceive and believe what they do, so the only facts that hold of those things we take to be coloured will be physical facts after all. They will be expressible in the terms of the physical sciences alone. But are there purely physical explanations of why human beings perceive and believe what they do about the colours of things? If there are, then it must be acknowledged as a fact of the world that human beings do perceive colour and believe that things are coloured. Otherwise there would be nothing for the physical explanations to explain. But human beings' perceiving and believing things conflicts with the uncompromising physicalist view that nothing in the physical world perceives or thinks and the physical world is the only world there is. Even the more accommodating physicalist view of colours requires that those psychological facts be capable of purely physical explanation, so it must acknowledge it as a fact of the world that human beings perceive and believe what they do about the colours of things. The difficulty is that the purely physical vocabulary, which for the physicalist is adequate to express all the facts there are, does not contain the resources for expressing those very facts about human perceptions and beliefs that must be accepted as facts of the world if the more accommodating physicalist view of colours is even to be stated. In acknowledging those psychological facts which purely physical facts and laws are held to be sufficient to explain, we are countenancing more than the purely physical facts that allegedly exhaust the only world there is. If we restricted ourselves to the physical world alone, the very facts whose explanation is crucial to the more accommodating physicalist view of colours would vanish. They would not be part of the only world which for the physicalist is real. For the convinced accommodating physicalist this will seem like no difficulty at all. The psychological facts in question only seem not to be facts of the physical world because they are not expressed in purely physical terms. If they are re-expressed as
THE PHYSICAL WORLD 273 physical facts a physical explanation will be available to explain them. But the difficulty in its simplest form is that there is no way of expressing those very psychological facts in purely physical terms. There would perhaps be no difficulty if psychological statements could be translated into synonymous or logically equivalent physical statements. They would mean the same, and so state the same facts. But that programme of explicit translation has rightly been abandoned, and physicalism is no longer taken to depend on full semantic reduction. The physicalist idea is that all that is going on when human beings perceive and believe what they do about colours is that certain physical events and processes are occurring. That is all there is in the world to 'make' those psychological statements true. The physical truths are not equivalent in meaning to the psychological truths, but they state all there is in the world 'in virtue of which' those psychological statements are true. And they state only physical facts. Physical explanations can be given of why those facts hold. There is no question that a great many physical events and processes occur whenever human beings perceive and believe what they do about colours. We are physical beings, and a lot of physical processes are always going on. The question about the source of physicalism is what reason there is for thinking that they are all that is going on, that those are the only facts that hold. In particular, does the possibility of purely physical explanation provide reason to believe any such thing? It is not easy to understand the physicalist claim that what goes on when people perceive and believe what they do about colours is nothing but physical events and processes. Suppose we grant that someone's seeing yellow at a certain time and coming to believe that there is a yellow lemon in front of him is a particular, datable occurrence. And suppose we even grant a physicalism of events, according to which every event that occurs is a physical event. Then the event of that person's perceiving and coming to believe what he does is identical with some physical event. But it would be ludicrous to say that it is nothing but a physical event-that that is all that is going on. The whole point of the theory of the identity of psychological and physical events is to say that a single event has both kinds of characteristics. That does not conflict with the view that every
274 BARRY STROUD event is a physical event. An event is a physical event if predications in physical terms are true of it. And to say of an event that it is psychological or economic or political is to say that predications in psychological or economic or political terms are true of it. If predications of all those kinds are true of one and the same event-as they might well be-and if it is also a physical event, it would be absurd to think that the event is exclusively physical, that only the physical predications are true of it. What then could the physicalist mean in saying that someone's seeing yellow and coming to believe that there is a yellow lemon in front of' him is nothing but a physical event? I claimed earlier that even if every thing that exists is a physical thing it does not follow that every fact is a physical fact, that everything that is true of physical things is something expressed in the terms of the physical sciences alone. There would be no reason to think that physical descriptions give us more than only certain aspects of the world involving those physical things, not all of it. Similarly, even if every event that occurs has true descriptions in purely physical terms there is no reason to think that nothing else is true of those events, that the physical descriptions give us more than only certain aspects of the world involving those events, not all of it. Neither a world with no non-physical objects nor a world with no non-physical events amounts to a world with no non-physical facts or truths. And facts or truths are what get explained. Physical laws and physical facts can be appealed to to explain physically-described phenomena. But what needs a purely physical explanation on the accommodating physicalist's view of colours is why human beings perceive and believe what they do about colours. If a person's seeing yellow at a certain time and coming to believe that there is a yellow lemon in front of him is a physical event, then there is a physical statement asserting that an event of a certain kind occurs at that time. That statement could ideally be derived from physical laws and other physical truths about the conditions in effect at the time. If we knew those laws and those conditions we could explain and understand in purely physical terms why an event of that physical kind occurred. But if we stayed within that purely physical story we would not have explained or understood why the person saw yellow and came to believe that there is a yellow
THE PHYSICAL WORLD 275 lemon in front of him. In fact, if we stayed within that physical story we would not even know that there was a person and the the person saw yellow and so on. No mention of such human or psychological phenomena even appears in that physical story. We would not know whether any of the events we had appealed to or physically explained were psychological events at all. Even if we were told independently-and of course in non-physical terms-that one of the events in question was a psychological event, we would not know on the basis of the physical story alone which one it was, or what kind of psychological event it was. It would be no better if we started from the other direction and concentrated on the person's seeing yellow and coming to believe that there is a yellow lemon in front of him, and then sought an explanation of it in physical terms. Suppose we were even handed a full description of the exclusively physical goingson and full physical explanations of why those physical events occurred. We would still be in no position to explain why that psychological event occurred. We would not even know which of the many physically-explained events it was, even if we knew in general that every event is a physical event and so we knew it had to be one of them. What we have an explanation for depends on the explanandum; what matters is the truth to be explained. This implies no limitation on the explanatory powers of the physical sciences. They can be said to be capable, ideally, of explaining everything that goes on in the physical world. But 'the physical world' here means the world as described in the terms of the physical sciences. Within that set of facts, the physical sciences can be as successful as you like in explaining everything they are supposed to explain. But their universal success in that domain does not show that there are purely physical explanations of psychological facts such as human beings' perceiving and believing what they do about colours. Such facts do not belong to the physical world in that sense. They do, of course, belong to the physical world if 'the physical world' means the world the physical sciences describe. Everything belongs to that world. That world is just what is described by everything that is true, not just by what is expressed in the terms of the physical sciences. It must be granted that the physical sciences do not, even ideally, explain everything that goes on in
276 BARRY STROUD the physical world in that sense. They do not physically explain the truth of everything that is true. But that is no limitation of the physical sciences. Physicalism is the thought that the facts expressed in physical terms, in the terms of the physical sciences alone, are somehow the only facts there are. The physical world is the only world. I do not think the admitted explanatory success of the physical sciences can be what leads to that thought. To suppose that physical explanations can be given of everything that is the case, because everything that is the case is somehow really a physical fact after all, is perhaps an expression of that physicalist thought. But it does not give us a reason for accepting that idea in the first place, nor does it explain what conception of the physical world is at work in it. Universal physical explanation seems possible to the physicalist only because he has already got the idea that the physical world is the only world there is. In arguing that there are no exclusively physical explanations of psychological phenomena I have not denied that there can be scientific explanations linking what we think of as the physical with what we think of as the psychological. But even if there are, the difficulty for physicalism remains. Any psycho-physical science would need laws or general statements connecting the physical and the psychological. They could not be expressed in purely physical or in purely psychological terms. Both sorts of terms, and therefore both sorts of facts, would be essential. The truth of any such laws would therefore conflict with an exclusive physicalism. It would imply that there are psychological phenomena, psychological facts of the world. The hope for some higher-level, purely physical laws which explained why those psycho-physical connections held would be open to a version of the earlier difficulty. An exclusive physicalist who appealed to such higher-level physical laws could not even acknowledge as facts the very facts of psycho-physical connection which those laws would be appealed to to explain. But to rest content with psycho-physical laws, with no higher-level laws to explain them, would be to rest content with a combined physical and psychological world, not an exclusively physical one. The thought or the hope that there could eventually be a single, unified science that could be used to explain everything that is the case is almost certainly one of the sources of
THE PHYSICAL WORLD 277 physicalism. But physicalism needs more than that. It needs the idea that that science will be an exclusively physical science. The difficulty mentioned and set aside earlier now seems crucial to the understanding of a science-based physicalism: what makes a science a physical science? Without an informative answer to that question it looks as if what has been called physicalism would be no metaphysical doctrine about the character of reality, or about what the world is like. It would just be the scientistic faith that eventually everything will somehow be scientifically explained. But even if we had an independently specifiable idea of the physical, and so could say what makes a science a physical science, we would still need some reason to think that facts expressed in the terms of such a physical science are the only facts there are.