Naturalism. Naturalism is the first assumption of ontological philosophy. It is the belief that the world is just the natural world. By the "natural world," I mean the world disclosed to us by perception, the world where we find ourselves, each having a body alongside others as parts of a world of objects in space that move and interact over time. That is the world of our daily lives.
It is the world to which we are all referring when we speak to one another, as language-using animals, about ordinary matters. We refer to objects in space, attribute properties and relations to them, and explain what happens to them. But some of the objects in space are also subjects, like ourselves, and we describe them in a special way. To them we attribute intentions, desires, thoughts, beliefs, perceptions and other subjective (or psychological) states. They are known by reflection, rather than perception (though knowing about the subjective states of others usually depends on perception as well). But that does not mean that subjective states are not parts of the natural world. They are parts of the natural world because they are states of beings like us, who exist as animals in the natural world. The natural world includes, therefore, not only what is known by perception, but also what is known by reflection.
What exists. The role of naturalism in ontological philosophy is to identify what needs to be explained, and for that purpose, it is appropriate to understand it in terms of its implications about what exists and what does not exist.
Positively. Positively, naturalism is the belief nothing exists but what is located in space and time. All the objects we perceive are located in space. Indeed, they are all related to one another as parts of a single world, since all the locations in space are connected to one another continuously in three independent dimensions. But objects can also move and interact with one another, and the events involving them are also parts of the same world, because all moments in time are connected continuously in a single dimension.
Though naturalism assumes that whatever exists is located in space and time, that does not mean that whatever has a location in space and time exists. Though events in the past and future have locations in space and time, they may not exist. Whether they do or not depends on how we resolve a profound ontological issue about the relationship between existence and time. We must decide whether to believe that existence itself is in time, so that only the present moment exists (or "presentism"), or to believe that time is just another kind of relation, like space, which holds among the things that exist. (See Ontology: Temporality and Spatiomaterialism: Time.)
Negatively. Space and time are so inclusive that naturalism may seem to be obviously true, but the significance of this assumption comes into better focus when we consider it negatively. For naturalism is also the denial that anything exists outside space or time.
God. God, for example, is supposed to exist outside both space and time. That is, at least, what traditional theists (and deists) must hold, for they believe that God is the creator of the natural world. (Nor is God part of the natural world by virtue of being ubiquitous, for that means existing everywhere in space at once, and if that were how God exists, He would be space.) Belief in a creator-God is a kind of supernaturalism. In fact, that is what was being scorned by those who first called themselves "naturalists" in the eighteenth century. They expected to be able to explain everything in the world without appeal to anything outside nature, and that negative sense of "naturalism" is what is intended here.
Forms. It is not just God, however, that naturalism denies. Neither are there any Platonic Forms. Plato held that there are objects knowable only by reason, such as mathematical objects, justice, and the nature of human beings, and even The Good Itself, which exist independently of the natural world. By that he meant that they existed not only outside space, but also outside time, for he he described it as a Realm of Being, opposite in nature from the Realm of Becoming, or nature.
Plato's main reason for postulating the Forms was to explain the nature of goodness objectively. He held that all the other forms follow from The Good itself, making them, and what participates in them, good. But this motive for believing that something exists outside space and time now generally takes the form of the belief in a supernatural God. Platonism is still defended, however, in the philosophy of mathematics. For example, numbers are supposed to be abstract objects. But since what makes them abstract is that their existence is not supposed to depend on anything located in space and time, naturalism must deny their existence.
Minds. Minds are also denied by naturalism, if they exist outside space, as the tradition of modern philosophy would have it. Though Descartes assumed that minds are in time, he denied that they are in space. (He argued that mind has a unity that precludes its being extended, which he took to be the essential property of objects in the natural world. Thus, he believed that mind is an opposite kind of substance from body, with mind and body existing independently of one another.) Insofar as minds are supposed to exist outside space, naturalism must deny their existence.
Problems. Naturalism holds, therefore, that there is nothing to be explained but the natural world. However, that does not mean that it can simply deny the existence of Cartesian minds, Platonic Forms, a transcendent God, and whatever else is supposed to exist outside either space or time. Naturalism must explain everything in space and time, and in each case, certain natural phenomena have led people to believe in the existence of these supernatural entities. Though those phenomena may depend on reflection, not just perception, they are clearly part of the natural world, for they occur to subjects like us in space and time. Thus, like everything else in space and time, they need to be explained.
Consciousness. What makes the mind seem immaterial is consciousness, that is, the way in which whatever we experience has an appearance to us. When we perceive a green leaf, for example, the color of the leaf has a certain intrinsic quality, and even though that quality seems to be located in the leaf, it has an appearance to us which we could not explain to someone who was blind from birth. The same holds not only for other colors, but also for sounds, odors, tastes, and bodily sensations of all kinds. These peculiar objects of reflection are called "phenomenal properties," "qualia," "raw feels," or the like, and they abound in normal perception. In perceiving the leaf, for example, we see many green qualia as covering its surface along with color qualia of other kinds on its stem and other nearby objects. Other kinds of sensory qualia seem to make us aware of its odor, its coolness, its taste, and the like. Each simple phenomenal property seems of have a certain location in space relative to the others at the time, and in the case of bodily sensations, such as itches and pains, they seem to have a locations in some part of the body which, in turn, is located in some part of the same phenomenal space as other objects of perception. Much the same kinds of appearances occur to us in remembering, imagining, and any kind of thinking about objects in space, though they are fainter, less distinct, and not always as spatially coherent. Indeed, even emotions, abstract thoughts, and other mental events have appearances for the subject to whom they occur.
This fact about experience is what will be meant here by "consciousness." Consciousness can make it seem that the conscious subject is not just an object in space, not merely a body alongside other objects in space, because each subjective state involves the appearance of many different kinds of qualia (or simple phenomenal properties) to the subject at the same time. This is the unity of mind to which Descartes pointed in order to show that mind is a basically different kind of substance from body. It means that mind cannot be cut up or divided into parts like extended objects in space. In other words, consciousness is not located in space, like a material object, but rather seems to contain a space of its own, because each sensory qualia appears to have a spatial location relative to all the others, as in the colors that appear to be on the surface of the leaf or its stem. Descartes called these appearances "ideas" and the subjects to whom they appear "minds," but the natural phenomenon to which he was pointing is the fact that there are such appearances to beings like us: qualia of many kinds all have locations in a phenomenal space, which is distinct from the space in which material objects exist.
The essential difference between mind and body led Descartes to believe that mind is a substance that is not located in space at all. Being indivisible, mind could not be part of extension, and thus, it was supposed to be an immaterial substance. Naturalism must deny that there are any minds in that sense. But to be credible, naturalism must somehow explain consciousness as a natural phenomenon. For we are certainly parts of the natural world, and it is hardly plausible to deny that we are conscious.
Goodness. If naturalism could explain consciousness in beings like us, it might seem that there would be nothing left to explain about Platonic Forms, because the abstract objects that appear to the experiencing subject in reasoning could be explained in the same way as ideas in the mind. (An explanation of abstract entities is, in any case, rightly demanded of naturalists, and brief statement of the one given here can be found in Relations: Ontological theory of mathematical knowledge.) There is, however, another aspect of the phenomena that led Plato to believe in Forms that would remain unexplained. Plato believed in the existence of Forms not merely because they are objects of rational intuition, but also because he believed that they are ideal and that things in nature are striving to be like them. That was his theory about the nature of goodness. Just as we try to be virtuous human beings, natural objects strive to be like their Forms, because the Forms are good.
Not only Platonists believe that there is a real difference between good and bad. It seems obvious to many people that goodness is something about the object, state, or event that makes it so that it ought to exist, whatever we may believe about it. For example, what makes an action morally right or wrong for beings like us is something about the action itself that makes it worth choosing, not just something we may believe or feel about it. Thus, goodness is also an aspect of the world that naturalism must explain.
The first attempt to explain goodness naturalistically was made by Aristotle. He thought that every natural object (as opposed to artifact) changes on its own for the sake of attaining an end, or final state, which is the fullest actualization of its essential form, and he explained this phenomenon by holding that there are "final causes" at work in the natural world along with efficient causes. For example, the acorn grows into an oak tree because the final cause of its natural kind is to be a mature oak tree. Growth and development are due to what is called "final causation." Aristotelian teleology, as it is called, explains how goodness is something objective by postulating a special kind of "force" in nature.
The belief in final causation was decisively rejected by most naturalists with the rise of modern science in the Renaissance. Modern science began with the discovery of laws of nature by which events in nature can be predicted, and explanation by such efficient causes was so obviously explanatory that, by contrast, explanations by final causes had to be rejected as merely descriptions of phenomena which call for explanation by efficient causes. Thus, teleology was rejected by naturalists. Nor could they reconcile the belief in final causes with their new found mechanism by holding that natural objects are designed to work mechanically toward certain ends, because that way of explaining the objectivity of goodness required them to believe in a God who created the natural world.
Many naturalists believe such that a naturalistic explanation of the difference between good and bad has been given by Darwin's theory of evolution. Darwin showed how organisms acquire traits that seem to be directed toward ends as a result of the natural selection of random variations on their heritable traits as the organisms succeed in reproducing. That explains why organisms seem to be changing in the direction of ends which are good for them. Thus, the difference between good and bad does not depend on how we feel about it. And Darwin's explanation involves only efficient causes. Thus, it is sometimes seen as the reduction of teleology to efficient causes.
However, most of those who believe that there is a real difference between good and bad and between right and wrong are not satisfied with the Darwinian explanation because of its accidentalism. As contemporary Darwinists understand it, natural selection is caused by external changes in the environment, which are inherently unpredictable, and that makes what evolves far too accidental to explain the difference between good and bad that is objective in the sense that they mean. (For a discussion of its accidentalism, see Change: Accidentalism.) They will insist that there is more for naturalism to explain about this phenomenon before they will be convinced that the world is just the natural world. Teleology is, therefore, still a problem for naturalism.
Holiness. Again, however, it might seem that if naturalism could give an adequate explanation of the objective difference between good and bad, it would not be necessary to explain the belief in God. God has been the traditional foundation for explaining why good is different from bad, for it is supposed to come down to his inscrutable purpose in creating the natural world. But even if there were a naturalistic explanation of the difference between good and bad, many who believe in God would not be satisfied, because what they believe in is not just that there is an objective difference between good and bad. They also believe that there is something worthy of worship, something so inherently good that we ought to accept it as the highest good, submit our wills to it, and treat it in a uniquely reverential way, that is, as something sacred or holy. The faithful believe that they have experiences of a kind that reveal the actual existence of such a thing to them, and the universality of religion among the cultures of the world makes this a phenomenon that must also be explained by naturalism, even though it denies there is any God existing outside space or time.
One way for naturalists to explain consciousness, the belief in a real difference between good and bad, and the sense that there is something in the world worthy of worship is to deny the reality of these phenomena. Naturalists can hold, in other words, that their critics are simply mistaken in how they describe these phenomena -- that what is being referred to is something quite different from what they believe.
Consciousness might be dismissed as a belief that results from a linguistic confusion (such as the belief in a "private language" or the acceptance of "folk psychology").
The belief in a real difference between good and bad might be explained away as a mere projection of our subjective feelings onto the world (in much the same way as objects in nature seem to have the colors and other phenomenal properties that are just ideas in the mind).
And the belief in something worthy of worship might be explained as simply what is feels like to submit to a higher authority.
Naturalists have given such explanations in the past. But they have not convinced those who take these phenomena to be real, and thus, naturalism has rightly been treated as just one possible view of the world among others. Though naturalism may be plausible to many people without an adequate explanation of these phenomena, there is good reason to doubt its truth as long as these explanations are not accepted as adequate by those who appeal to these phenomena. Theists, mind-body dualists, and those who believe in objective goodness are rational beings too, and if naturalism is a reasonable view, it should be reasonable to them. Thus, the burden that naturalism must bear is rather large. It must be able to explain everything in the world, including these problematic phenomena, to the satisfaction of every rational being, including those who have been led to believe in entities existing outside space or time — that is, as long as they are willing to give reasons and not just be arbitrary and dogmatic in their assertions about what exists.