Contemporary philosophy. Naturalism is the attitude of contemporary philosophy. In the twentieth century, continuing advancement by science in explaining the natural world, discovering laws of nature and various mechanisms embodying them, made the abstruse and inconclusive arguments of philosophy of modern philosophy seem fundamentally misguided. Philosophers abandoned the Cartesian method and its metaphysical problems in favor of an explanation of how we know that derives from reflecting on knowledge as an intersubjective process, and that brought with it a commitment to naturalism. And contemporary philosophers accepted natural science, with some reservations, as the most adequate way of knowing we have. Thus, the problem that mind poses for contemporary philosophers can be seen as a question about how a science of consciousness is possible. Contemporary philosophers assume, as naturalists, that what modern philosophers called "mind" must somehow be part of the natural world, and though they could dismiss mind-body dualism, it was harder to deny the difference between physical and phenomenal properties. Those who affirm the existence of phenomenal properties as well as physical properties are called "property dualists." For naturalists, therefore, the question became how phenomenal properties can be included as something characterizing the natural world being explained by science, even though science refers only to physical properties.

For present purposes, let us take "physical properties" to include functional properties, such as "being a clock" or "conveying signals." They may not be reducible to physical properties, but since no one denies that they "supervene," at least, on physical properties, all the causal connections in particular cases come down to basic physical properties. And the issue is how phenomenal properties are related to physical or functional properties.

Contemporary philosophers have taken great care to show that phenomenal properties are different from physical properties, for example, in famous arguments by Thomas Nagel, Frank Jackson, and Saul Kripke. By asking what it is like to be a bat, Nagel (1979, 1986) was pointing to a subjective aspect of experience that cannot be known by the "view from nowhere", that is, by natural science. Jackson (1982) made it clear that phenomenal qualities, or qualia, are themselves objects of knowledge by pointing out that Mary, a neurophysiologist who studied the physical mechanism of color perception in a laboratory devoid of red objects, would come to know something more about the perception of red when she left the room and actually saw something red, namely, how red appears to the subject. And Kripke (1980) showed that properties rigidly designated by how they appear to subjects cannot be identical to physical properties because the connection is not metaphysically necessary, as it would have to be, if they were identical.

For those who are inclined to take natural science as revealing the basic nature of the world, the problem of mind is how there can be a science of consciousness. It is most obviously problematic when science is understood as using a method that bases its conclusions on observation in one way or another. This reliance on observation is a basic tenet of its empirical method as traditionally understood, for example, by empiricists, logical positivists and most practicing scientists. (Though there are well known problems in the philosophy of science about the theory-ladenness of observation statements, it is agreed on all sides that observation depends on perception, that is, on the use of our sensory organs to discover the states of objects in space.)

The reason that this tenet of the empirical method makes consciousness a problem for science is that phenomenal properties are apparently knowable only by reflection. We have seen how the difference between physical and phenomenal properties was discovered -- that is, by reflecting on the causal explanation of perception from the point of view of the perceiving subject. But it also seems that our only "evidence" that psychological states involve phenomenal properties comes from each of us reflecting on our own psychological states. The nature of simple qualia, for example, what red qualia are like, is not revealed to observation. They are private to each individual subject.

There are ways of observing the brain in operation, and new ways are being developed. But no one has found a way of using such observations to demonstrate that brain states involve phenomenal properties. Indeed, neurophysiologists don’t expect their methods ever to show either the existence of phenomenal properties or how qualia appear.

To be sure, there is evidence for the existence of phenomenal properties in what people say about their psychological states. But that evidence depends on scientists interpreting the other’s talk of qualia and phenomenal properties as references to objects of the same sorts they each know privately by reflection on their own phenomenal properties. The verbal behavior itself does not seem to depend on anything but physical causes.

The difference between reflection and perception makes it doubtful, therefore, that science will ever be able to know about phenomenal properties.

Anti-realism about phenomenal properties: eliminative materialism. One quick way of dealing with this problem is simply to deny there are any phenomenal properties. This is, in effect, anti-realism about phenomenal properties from the point of view of science, though it is usually called "eliminative materialism," by the kind of ontology it defends.

In one version, eliminative materialism holds that the need for talk of phenomenal properties will eventually be eliminated, at least from science, as science explains all the phenomena relevant to psychology in its own terms. That would show that our traditional talk about phenomenal properties (and psychological states, such as perceptions, beliefs, desires, and the like) is just a mis-description of what really exists, which is fully described by physical properties. (See Churchland 1995.)

It is also possible to argue that we are fooling ourselves to think that traditional talk about phenomenal properties is meaningful in the first place. (See Dennett 1991 and Rorty 1979.)

But eliminative materialism does not show how a science of consciousness is possible. Rather, it holds that a science of consciousness is not necessary because there is nothing to be explained. The problem of mind arises only for those who are realists about phenomenal properties and believe that they exist in addition to physical properties.

Realism about phenomenal properties: property dualism. During most of the century, empiricism in psychology took the form of behaviorism, the attempt to explain human beings in terms of laws describing their observable behavior. Consciousness was thereby banished from science. But that is puzzling to contemporary naturalists, for they expect natural science to explain everything in the natural world, and they know, as reflective beings, that they themselves are conscious. They are realists about phenomenal properties, and that makes them property dualists, because they recognize the existence of phenomenal as well as physical properties. And the problem of mind can be seen at the attempt to show how science can study consciousness, that is, how it can justify theories that refer to the phenomenal properties of psychological states. There are several possibilities.

Emergentism. The most obvious way for science to include consciousness would be to take mind to be an immaterial substance that is located in space along with bits of matter. Or if we call everything located in space "matter," it is to hold that some bits of matter have phenomenal properties that play a causal role in the natural world. If phenomenal properties of bits of matter did somehow make a difference to what happens in nature, they would be not only effects of physical causes, but they would themselves be efficient causes, and their existence could be detected empirically. Science could know about them in the same way it knows about other unobservable entities, such as electrons and force fields. Bits of matter with phenomenal properties would have to be mentioned by the best explanations of what can be observed through perception alone.

It is conceivable, at least, that phenomenal properties would have to be introduced by some branch of science, such as neurophysiology. The mechanisms found in the brain might provide no way of explaining, for example, why human beings say that they have phenomenal properties or why they call certain sensations green and others red. If all possible physical explanations were ruled out, the best explanation might be to hold that reports about phenomenal properties depend causally on how psychological states appear to the subject having them, which would mean that phenomenal properties are efficient causes. Phenomenal properties would then be unobservable entities of neurophysiology.

Any such neurophysiological discovery would, however, have serious implications for physics. The grounds for believing that there are phenomenal properties playing a causal role would be that no physical mechanism can explain certain verbal behaviors, and that would imply that there are efficient causes at work in brains that are not physical properties. This would be shocking, for physics is thought to be causally complete, in the sense that physical properties are sufficient, in principle, to explain every kind of event that happens to what is located in space.

It might be argued that the reason physics has not noticed the causal role played by phenomenal properties is that they are emergent and make a difference only in highly complex physical objects, such as brains, which evolve (or in complex functional systems generally). But in order for phenomenal properties to be effective in brains, neurophysiology would have to predict something different from what physics would predict for the same situations on the basis of physical properties. Thus, physics would have to come to believe that some material objects have properties in addition to the physical properties that it has already recognized and that these new properties affect how physical entities move or interact in certain situations. In other words, this kind of emergentism would be causal. Such a discovery would contradict physics as we know it. At a minimum, it would show that physics is not causally complete.

A science of consciousness could, therefore, be established by a scientific discovery of the kind that even the most hidebound defender of the traditional view of the empirical method could not deny. That would be a scientific solution to what has heretofore seemed to be a philosophical problem about mind. There is, however, no evidence at present suggesting that phenomenal properties should be introduced as unobservable (that is, not directly perceivable) theoretical entities of neurophysiology. It seems quite unlikely to contemporary naturalists, considering how radically physics would have to be mistaken. And if phenomenal properties are, as ontological philosophy suggests, the intrinsic essential properties of certain kinds of matter involved in the function of the brain, they have no causal role. All the causal roles are played by extrinsic essential properties, that is, the physical properties already recognized by science.

Epiphenomenalism. Another way founding a science of consciousness would be to accept reflection as a form of observation in science. Though reflection has long been the province of philosophy, this avenue is open to naturalists who think of philosophy as "continuous with" science.

This trend in recent philosophy of science explicitly abandons epistemology in the traditional sense of providing an a priori foundation for the justification of science and its method (Kitcher 1992; Rosenberg 1994). Instead of "first philosophy," it proposes to use the results of science itself to justify and improve the methods of science, which has given it the name "naturalized epistemology" (after Quine 1969). For example, scientific discoveries about the mechanisms of human cognition could be used to improve evidence gathering methods in science as much as discoveries about the accuracy of any measuring instrument. But those same human beings have a capacity for reflection as well as perception, and thus their reflection on phenomenal properties could be considered a way of gathering evidence about the natural world which is just as legitimate as their perception of physical properties. To naturalists of this kind, therefore, it may seem there is no obstacle to a science of consciousness. Indeed, these days, cognitive scientists often use reports about reflection on phenomenal properties as evidence, a practice recently defended by Goldman (1997).

To recognize reflection, including what can be known only by reflection, as part of the data base of natural science is, however, a trivial solution to the problem of mind. It overcomes the epistemological obstacle to a science of consciousness by, in effect, redefining "science" to include a form of knowledge that has traditionally been taken as the foundation of by philosophy.

It will not be acceptable to naturalists who cleave to a more traditional notion of empirical science as based on observation by perception. They will dig in their heals from fear of opening the door to other forms of private knowledge in science, such as the intuitions by which rationalists justified their metaphysical systems. And attempts to draw a new line of demarcation between science and philosophy that will include reflection on phenomenal properties but exclude the supposed certainty of clear and distinct ideas would seem like mere gerrymandering.

Even if there were no epistemological objections to reflection, however, this avenue to a science of consciousness would lead to ontological problems for science. It would complicate the scientific view of the natural world in a way that is quite problematic, for it would be to acknowledge the existence of properties that simply do not fit together intelligibly with the properties already recognized by science. The latter come down to properties mentioned by physics. Specifically, physical (and functional) properties seem to be responsible for all the behavior and internal processes found in complex organisms like us. Thus, to acknowledge the existence of phenomenal properties on the grounds that they can be "observed" in nature through reflection on what experience is like would be to recognize that some natural objects, human beings, at least, have properties of a fundamentally different kind from those already recognized by natural science. And if physicists are correct in believing it to be possible, in principle, to explain everything that happens in nature by the efficient causes picked out by physical properties, two facts about these properties follow. One is that phenomenal properties are somehow effects of the physical (or functional) properties of such organisms. The other is that having phenomenal properties cannot itself have any effect, in turn, on physical or functional properties. In other words, phenomenal properties would be epiphenomenal relative to physical (and functional) properties.

Epiphenomenalism is, at best, an inelegant ontology. It takes phenomenal properties to be "nomological danglers," in Feigl’s (1958) famous terms. Epiphenomenalists can insist, of course, that there is a causal necessity about the connection between physical (and/or functional) properties and phenomenal properties. But it would be just an assumption, for they have no explanation of why physical (or functional) properties give rise to phenomenal properties. Nor any explanation of why phenomenal properties should be impotent.

Thus, if the goal of science is to discover all the most basic laws of nature, epiphenomenalism would mean that those most fundamental laws include not only the basic laws of physics, which describe efficient-cause connections, but also psychophysical laws, which describe a regular connection between physical (and/or functional) properties and phenomenal properties. (For example, see Chalmers1996, pp. 87, 170-1, 274-5.)

Or, to use Kripke’s (1980, p. 153-5) famous metaphor, God, in creating such a world, would have to go back, after creating all the physical objects and putting them together as a natural world, and tack on the phenomenal properties. The extra effort required belies their odd status. No one finds epiphenomenalism satisfactory. (It repels even Chalmers 1996, p. 160.)

Necessary connection between physical and phenomenal properties. Ontological philosophy provides, as we have seen, a way of avoiding the problem of epiphenomenalism. Though it accepts property dualism, it reveals a necessary connection between physical and phenomenal properties, and that would found a science of consciousness, because it would show that phenomenal properties are already part of what exists according to science. Contemporary philosophers recognize that demonstrating a necessary connection between physical and phenomenal properties would solve the problem with epiphenomenalism (and thus, the most basic aspect of the problem of mind), but they have not been able to take this avenue all the way to a science of consciousness, because cannot see how it is possible to show that phenomenal properties are necessarily connected to something science already mentions in its physical (and/or functional) descriptions. The obstacle they encounter comes from the epistemological approach to philosophy, which contemporary naturalists have inherited, for in this case, ontology as mere realism makes it seem that properties are more basic than substances. Let us see how they fail to find any way to demonstrate a necessary connection between physical and phenomenal properties before we compare epistemological to ontological philosophy.

Necessity in Epistemological Philosophy. Contemporary analytic philosophy offers various ways in which a necessary connection might be established. Let us consider them.

A priori necessity. The original form of necessary truth in contemporary Anglo-American analytic philosophy was analytic truth, or propositions that are true by virtue of the meanings of the terms involved. That would bean a priori connection between physical and phenomenal properties, but it is not a possible foundation for a science of consciousness, for the inability to see an intelligible connection between them is the very problem of consciousness. What we mean by "phenomenal properties" is so different from what we mean by "physical properties" (or by "functional properties," for that matter) that it seems almost absurd even to compare them. That makes it easy to conceive of possible worlds that are physically like our own, but which lack phenomenal properties altogether. That is, there could be a world of zombies, or beings that are physically and functionally indistinguishable form us except for not being conscious. It is also possible to conceive of worlds with phenomenal properties but no physical properties, for that is the view that was defended in modern philosophy as idealism. Hence, no necessary connection can be established a priori.

Causal necessity. Any necessary connection between physical and phenomenal properties must, therefore, be a posteriori. It must be something we can somehow discover about the world from experience. But it cannot be a mere causal necessity of the sort that laws of nature are supposed to have. That sort of necessity would reduce either to causal emergentism or to epiphenomenalism, depending on which causal connections phenomenal properties were supposed to have (that is, being effects of physical properties that are also causes of them, or else being effects that are not causes). It is their inadequacy that forces naturalists to look for a metaphysically necessary connection.

Theoretical identification. The more popular model for discovering necessary connections is theoretical identification in science, such as the discovery that water is identical to masses of H₂O molecules. Thus, just as the solidity of ice was discovered to be identical to the stability of the crystal structure formed by weak hydrogen bonds among adjacent H₂O molecules when their kinetic energy fall below a certain point, so phenomenal properties might turn out to be identical to physical properties of some other kind.

However, physical and phenomenal properties cannot be related in this way, because theoretical identification is a necessary connection. As Kripke(1980) showed, in order for two (rigidly designated) properties to be identical, it must be impossible to conceive one without the other. For example, if the solidity of ice is identical to a certain kind of crystalline structure of H₂O in the actual world, then the identity must hold in any possible world where either exists. It is not, however, impossible to conceive of worlds in which beings physically and functionally like us lack phenomenal properties altogether. No scientific theory can identify the two kinds of properties, and so a world of zombies is still possible.

Supervenience. If the reduction involved in theoretical identification does not provide an avenue to a science of consciousness, science does not offer many other models for showing a necessary connection between physical and phenomenal properties. One possibility is supervenience, which is a weaker relation than the complete reduction involved in the theoretical identification of apparently different physical properties. What has forced philosophers to recognize supervenience is the existence of functional properties. Though a functional property may be identical to certain physical properties in particular cases or classes of cases, there are many other ways that the same functional property can be realized by physical properties and, thus, no general identity between properties at the two levels. For example, there are many kinds of physical mechanisms that can function as clocks. And physical properties that do are said to "realize" a clock. But supervenience cannot be how phenomenal properties are related to physical properties, for that would require phenomenal properties to be identical to physical properties in particular cases, and that is what does not seem to be the case. Thus, a zombie world still seems possible.

A process of elimination leads to the conclusions that, if there is a metaphysically necessary connection between physical and phenomenal properties that can be discovered by experience, it must a new kind of relationship, not previously recognized by science. That is what ontological philosophy offers, and though it is beyond the reach of epistemological philosophy, David Charlmers comes close.

Chalmers (1996, p. 135) considers the possibility that "there are properties essential to the physical constitution of the world that are not accessible to physical investigation." The existence of such intrinsic properties is plausible to him, because all the properties mentioned by physics are basically relational, characterizing entities by their causal connections and other relations to one another. Even physical properties that seem to be inherent in the objects that have them, such as mass, energy, spin, and charge, are measured by the causal relations they have to one another. Thus, whatever has physical properties could also have an intrinsic nature.

However, Chalmers has no way to understand how they might be related to physical properties, because he does not think of substances as anything more than the properties they have. That makes properties ontologically basic, and so he tries to describe the relationship by saying that intrinsic properties might " ‘realize’ the extrinsic physical properties, and that the laws connecting them might realize physical laws" (155). And describing the significance of discovering some such relationship, he says that, if intrinsic properties were "constitutive of physical properties" (136), then even though a zombie world may seem to be physically identical to ours, it would actually be different physically, for it would lack some "inaccessible essential properties, which are also the properties that guarantee consciousness" (135). This is the view of phenomenal properties to which Chalmers himself inclines (153-156), though it has also been suggested by others.[1]

As Chalmers recognizes, however, to suggest that intrinsic properties are a special kind of phenomenal (or proto-phenomenal) property underling all physical (and functional) properties is not to show that there is a necessary connection between intrinsic and physical properties. It is merely to point to a possibility. Chalmers (135) rightly calls it "speculative metaphysics." Though it may be coherent, it is no more than speculation, because without the concept of substance to explain the nature of properties, it is just a vague possibility. And since nothing makes it inconceivable that a world physically like our own would lack intrinsic properties, this view reduces to property dualism — a point that Chalmers makes by invoking Kripke’s metaphor: "After ensuring that a world identical to ours from the standpoint of out physical theories, God has to expend further effort to make that world identical to ours across the board" (136). Zombies are still possible.[2]

Metaphysical Necessity in Ontological Philosophy. What keeps epistemological philosophy from discovering a necessary relationship between physical and phenomenal properties that would found an empirical science of consciousness is the implicit assumption that properties are basic. What enables ontological philosophy to show that phenomenal properties are an essential part of the natural world investigated by science is reducing properties to substances. Physical properties, as we have seen, characterize the extrinsic essential natures of all forms of material substances, and if phenomenal properties characterize the intrinsic essential nature of some form of matter that helps constitute the conscious subject, phenomenal and physical properties would be related as intrinsic and extrinsic aspects of the essential natures of the substances constituting the world. That relationship does seem to be metaphysically necessary in the sense relevant in this debate, though in our terms it is an ontologically necessary truth, since the necessity of its truth comes from its being an implication of the ontology we have found to be true on empirical grounds.

Metaphysically necessary truths are understood as holding in every possible physical world, and the connection proposed by ontological philosophy is necessary in that sense, for it would hold in any possible physical world in which the basic laws of physics are descriptions of how elementary material substances move and interact. Their basically relational nature indicates that physical properties characterize the extrinsic essential natures of those substances. But since substances cannot have such properties unless they have some way of existing apart from the relations, they must also have an intrinsic essential nature. Thus, Zombies would be impossible. Any being with all the same physical (and functional) properties would necessarily also have intrinsic properties.

To use Kripke’s (1980, 153-4) vivid image, God would not have to go back and tack on intrinsic properties after he had created the physical world, for if God had created the world by combining many material substances in the first place, those substances would already have intrinsic natures of some kind or other. In fact, it would not be possible for God to create a physical world out of multiple substance without intrinsic properties, even if he wanted to.

This is not to say, however, that there is no possible physical world without intrinsic properties. It is possible for a world to have all the same physical (and functional) properties as our own and yet to lack intrinsic properties.

That would be the case, for example, in a physical world that is not constituted by substances at all, as the empiricists’ so-called "bundle theory" of substances would have it. (That is, however, just the form of idealism that one finds when one looks in empiricism for a theory of what exists.)

Even if the physical world must be constituted by substances of some kind in order to exist independently, it could lack intrinsic properties, for it could be constituted by substances that are mere substrata for physical properties (assuming that it is coherent to suppose there can be substances without any inherent properties at all).

Nor would intrinsic properties be needed if the world were constituted by a single substance in which particular properties have spatiotemporal locations.

The necessity of the relationship between intrinsic and extrinsic properties depends, in other words, on an ontological assumption that is not itself necessary, namely, that the world is constituted by many particular substances existing together in some way. There was no such condition on the kind of metaphysical necessity that Kripke discussed, for he was considering only the possibility of properties being identical. That is, if phenomenal and physical properties were identical, there would be no possible physical world without phenomenal properties. But the way in which ontological philosophy demonstrates a metaphysically necessary connection does not come from discovering the identity of two apparently different properties. It comes from discovering that material substances must have two aspects to the essential aspect of the nature as substances. That is, it depends on a theory about the nature of the substances constituting the world that can be justified empirically. (As we have seen, the foundation of ontological philosophy is established by accepting naturalism, taking ontology to be explanatory, and using the empirical method to decide which possible ontological explanation is true.)

Kripke's model for identifying properties with one another comes from discoveries in science that physical properties picked out on the macro-level (such as the solidity of ice) are identical to physical properties picked out on the micro-level (such as the hydrogen bonds among H₂O molecules under certain temperature and pressure conditions).

Ontological philosophy, by contrast, discovers how properties characterizing one aspect of the essential nature of substances (their extrinsic essential nature) are related to another aspect of the essential nature that such substances must have (their intrinsic essential nature). The nature of material substances is what connects them.

Moreover, this ontological explanation of properties is what realists about physics would have to accept, if they took up the ontological issue about their nature at all, for the assumption that there are substances whose aspects are properties is certainly more plausible than any of the alternative theories about substances: the bundle theory, the substratum theory, or the assumption that the whole world is a single substance. And if physical properties are simply the extrinsic essential aspects of the various material substances making up the actual world, naturalists will come to recognize that every possible physical world is made of multiple substances and, hence, that material substances have intrinsic properties in every possible physical world.

This ontological explanation of the necessary connection between physical and phenomenal properties is not a priori, but a posteriori, because it is discovered. As Kripke agues, that means that it must be possible for it to appear that there are possible world in which it does not hold. Kripke showed how such an appearance of contingency is caused in the case of theoretical identification. But it is also possible on this ontological explanation of phenomenal properties to explain how it is possible for it to appear that there are possible worlds in which this ontologically connection does not hold. The illusion of contingency about their relationship comes from failing to recognize that the physical world is constituted by multiple substances and seeing how properties are reducible to them. That is why Chalmers dismisses the belief in intrinsic properties as mere speculation.[3]

Relative to a necessary connection established by the identity of properties, the connection established by this ontological argument for its necessity is limited. From the ontological necessity of the connection between intrinsic and extrinsic essential natures of substances it does not follow that there is a ontologically necessary connection between phenomenal and physical properties, not even if phenomenal properties are a kind of intrinsic essential nature of certain substances in our world. Since intrinsic and extrinsic properties characterize different aspects of the essential aspect of substances, it is conceivable that in another possible physical world made of multiple substances, substances would have the same physical properties as ours, and yet have different kinds of intrinsic properties. That is, different worlds could be constituted by different kinds of material substances.

Thus, beings that are physically similar to us in another world constituted by multiple substances might have phenomenal properties with, for example, an inverted spectrum of color qualia. Or they might have more radically different kinds of intrinsic natures. All that is ontologically necessary is that beings like us physically in any possible world made of substances have intrinsic natures of some kind. Though a zombie world is not ontologically possible, an inverted spectrum world is.

Despite this limit to what is necessarily true, however, it is still possible to found a science of consciousness on this ontological explanation of properties, for it implies that, in any possible physical world made of the same kinds of substances as those constituting our world, there are no beings physically and functionally like us that do not also have phenomenal properties like ours. That is enough to found a science of consciousness, because our science is about the actual world. It would be gratuitous to hold that physically indistinguishable material substances in the actual world are different kinds of substances in this sense, especially since they are convertible into one another. Thus, the kinds of phenomenal properties on which one reflects will be the same as those on which other subjects reflect, if the relevant physical properties in the brain are the same.

This ontological explanation of phenomenal properties also explains how they are objects of knowledge. It phenomenal properties are the intrinsic essential nature of some form of matter making up conscious subjects, we can explain why there is something more for Mary to learn about perception when she leaves the black and white neurophysiology laboratory in which she has spent her life and finally sees something red. When she sees something red, the process she has been studying all her life is for the first time embodied in her. Some bit of matter that helps constitute Mary herself has an intrinsic essential nature of a kind whose extrinsic essential nature has been one of the objects of her study. Thus, Mary learns what it is like to be a certain bit of the matter involved in that process.

The property that Mary discovers is, however, an epiphenomenal property on this theory. If phenomenal properties are kinds of intrinsic properties, they are never the efficient cause of anything that happens in the world. The efficient causes are all properties characterizing the extrinsic essential natures of substances, and since they determine what happens, they determine the kinds of bits of matter that exist and, thereby, all the intrinsic properties in the world. But phenomenal properties are not mere "nomological danglers," because intrinsic properties earn their claim to reality for natural science by being necessary aspects of the same substances whose extrinsic essential natures are physical properties.[4]

Finally, this ontological reduction of properties also solves Nagel’s problem about the relationship between the "view from nowhere" and the subjective aspect of experience, or "what it is like." By the "view from nowhere," Nagel means the scientific view of the natural world, and if this ontological interpretation of physics is correct, that is the view of the world as being made up of material substances related spatially as parts of the same world. The problem, as Nagel sees it, is that the scientific view leaves out the subjective aspect of experience.

That problem is solved, however, if the world is made up of substances in the sense assumed here, for the subjective aspect of experience turns out to be the intrinsic aspect of the essential nature of certain elementary material substances making up the subject as an organism in nature. What is left out of the "view from nowhere" is not the existence of phenomenal properties, but only their nature. To know their nature, it is necessary to be the substances making up the subject, because what it is like for the subject is the kind of intrinsic essential nature of the relevant bit of matter.

It is still necessary, however, to explain another aspect of the nature of consciousness, namely, its unity, or why so many different kinds of qualia all appear to the same subject and that same time in perception. That is explained in Change: Unity of Consciousness. But that depends on the implication of spatiomaterialism for science, and before taking up science, we must explain why mathematics is true.