Before we can say what the metaphysical naturalist worldview looks like, we must first articulate the definitional limitations of what the ‘natural’ means. Philosopher Paul Draper defines naturalism as:
“The hypothesis that the natural world is a closed system [in the sense that] nothing that is not a part of the natural world affects it.”
This definition of naturalism is important to distinguish metaphysical naturalism from methodological naturalism. As philosopher Keith Augustine explains, metaphysical naturalism is a “substantial view about the nature of reality,” whereas methodological naturalism “is the principle that science and history should presume that all causes are natural causes solely for the purpose of promoting successful investigation. The idea behind this principle is that natural causes can be investigated directly through scientific method, whereas supernatural causes cannot, and hence presuming that an event has a supernatural cause for methodological purposes halts further investigation” . Augustine further elaborates:
“In utilizing methodological naturalism, science and history do not assume a priori that, as a matter of fact, supernatural causes don’t really exist. There is no conceptual conflict between practicing science or history and believing in the supernatural. However … methodological naturalism would not be as stunningly successful as it has in fact been if metaphysical naturalism were false. Thus the de facto success of methodological naturalism provides strong empirical evidence that metaphysical naturalism is probably true.”
As construed from above, metaphysical naturalism is not only the practice of looking for natural causes and explanations (which may be termed methodological naturalism), but also provides a “big picture” explanation of reality positing that nature and natural causes are all that exist.
So far so good, but more needs to be said now about what exactly we mean by the terms ‘nature’ and ‘natural.’ As Keith Augustine in A Defense of Naturalism explains:
“One obvious candidate for what is meant by the term ‘natural’ is physical. The earliest forms of naturalism, in fact, were versions of materialism or physicalism which maintained that everything that exists is physical. As I have construed naturalism, simple (reductive) physicalism maintains that everything that exists within nature is physical and solely influenced by physical causes.”
Physicalism is the view that all of reality can be described by physics and is reducible to physics. This is one candidate for what we mean by naturalism, but it need not be the only definition (as will be discussed below). Augustine goes on to explain:
“However, the prominent twentieth century debate over materialism in the philosophy of mind has revealed several difficulties with reductive physicalism as a solution to the mind-body problem. One of the most persistent difficulties for reductive physicalism has been the apparent inability of physicalistic explanations to capture qualitative features of conscious experience. It has been persuasively argued that qualia–the experiential feels of ‘what it is like’ to be in a conscious mental state–cannot be captured by any physicalistic explanations in principle because physicalistic explanations inherently refer to objective or public features of phenomena, whereas the experiential features of consciousness are inherently subjective or private.”
Indeed, explaining consciousness and subjective experience in terms of physics can be rather challenging, which raises a number of issues for a physicalist explanation of reality. However, these issues are not without solution. Augustine continues:
“In the contemporary philosophy of mind, an attractive alternative to reductive physicalism is some version of nonreductive physicalism or property dualism. According to nonreductive physicalism, mental states are not simply identical to certain physical states (such as brain states), as reductive physicalists hold; rather, mental states are supervenient upon those physical states … To say that mental states supervene upon physical states is to say that there can be no differences between mental states without a physical difference between the objects which instantiate those states … This physical difference usually amounts to a difference in brain states, though the same mental states may be supervenient upon the physical states of an advanced computer or of an extraterrestrial brain. For our purposes, it is sufficient to say that for a mental state to be supervenient upon a physical state entails that a mental state is dependent upon and determined by that physical state without necessarily being identical to it.”
All of this may seem rather dense, but the importance of positing that mental states are supervenient upon physical states is that no minds can exist without a physical state, which excludes ghosts, demons, souls, and an immaterial ‘G’od. Reframed in this way, Augustine refines the definition of naturalism to be:
“On my definition of naturalism, nonreductive physicalism maintains that everything that exists within nature is either physical or supervenient upon the physical and solely influenced by physical causes or causes which are supervenient upon physical causes. A more economical statement of this form of naturalism would drop the idea of supervenient causation: everything that exists within nature is either physical or supervenient upon the physical and solely influenced by physical causes. Most reductive and nonreductive physicalists alike subscribe to the causal closure of the physical–the view that all caused events in the physical world must have physical causes.”
In this way the mind-body problem can be understood to be compatible with and explicable under a physicalist view of naturalism. However, as I stated above, one does not need to define naturalism strictly by the physical. Rather, a more general definition can be applied to describe, what I would term, a ‘minimalist naturalism,’ i.e. a naturalism that cannot be reduced to any more simple description and thus encompasses the widest range of natural meaning.
Minimally, as I discussed in a previous blog, metaphysical naturalism is the view that there is no metaphysical teleology . Our ultimate reality has no intentionality, goal orientation, or agency. This follows from the standard meaning of when we say that something has a ‘natural explanation.’ Something that happens naturally occurs without any plan or design, but as the result of unintentional causes. The universe was not designed for anything nor is it moving towards anything. Naturalism predicts a ‘blind’ cosmos of the very sort we live in.
Such a universe would exclude a conscious, willful being designing the universe for a specific purpose (such as theism entails). But, how does this framework also exclude things like ghosts or even a guiding principle like karma? Naturalist philosopher Richard Carrier offers a more specific definition in his article “On Defining Naturalism as a Worldview“:
“Naturalism is true iff everything that exists is either ontologically reducible to the nonmental, or causally reducible to the nonmental, or both … For A to be ontologically reducible to B, there must exist nothing in A that is not made up of elements of B … For A to be causally reducible to B, it does not have to be ontologically reducible to B or to anything else, it only has to be entirely causally explained by B or some arrangement of B … A mental object is any object that is distinctive of the contents or activity of a mind, in contrast to what we do not consider as such. The most obvious examples of mental objects in this sense are thoughts, perceptions, and emotions.”
All of this, once again, is rather dense. However, what Carrier’s definition explains is why things like souls are incompatible with naturalism. Souls represent irreducible mental sates. They cannot be broken down into non-mental parts. In contrast, we can consider a mind dependent on a physical brain to be a mental state, but, unlike souls, a physically-dependent mind can be broken down into lesser non-mental parts.
Carrier further explains in “Defining the Supernatural” how this definition likewise excludes anything that we might term a supernatural being, power, property, or effect, all of which can be eliminated if the chain of ontology and causality is nonmental.
All of this is crucial to explaining why naturalism rejects religion. David Eller in The End of Christianity (pgs. 274-275) defines religion as the following:
“Religion is that worldview, that paradigm, which sees nonhuman/superhuman minds/wills/intentions at work and which ‘explains’ events and legitimizes relations and institutions in terms of these beings and their wills.”
Religion presents a view of the world that is deeply imbued with guiding teleologies and irreducibly mental states. Drawing from the minimalist definition of naturalism above, however, a universe with no irreducible mental states has no teleology in its metaphysics. Everything that is teleological within a naturalist universe must derive from non-teleological causes. Minds and willful intentions must arise from and be causally reducible to non-mental, impersonal, and, what we may call ‘natural’ states. Everything in a naturalist universe thus reduces to non-teleological and impersonal entities and causes.
This definition of both naturalism and religion is important, since, under such a distinction, metaphysical naturalism excludes all religions.
Thus, starting from a universe that reduces entirely to nonmental states, the subsequent posts in my series, “Thinking about the ‘Metaphysics’ in Metaphysical Naturalism“, will examine how naturalists can explain issues of cosmology, time, unguided evolution, free will, epistemology, analytical vs. synthetic knowledge, particulars and abstract objects, and ethics, under this definition of metaphysical naturalism, which has now been spelled out above.
 Richard Carrier in “Defining the Supernatural” provides a critique of the view that science cannot investigate (and thus verify or falsify) the supernatural under the sub-section “Is the Supernatural Knowable?”.
 In using teleology as a criterion to contrast the “natural” from “supernatural,” I am referring to the Platonic sense of teleology and not the Aristotelian sense. The Platonic sense of teleology is more akin to Paley’s teleological argument for God, which maintains that the universe reflects a willful process of design that points to an intelligent creator. The Aristotelian sense of teleology pertains more to substance being directed toward certain ends, which can be formulated as “having the potentiality to change in a predictable way or to move towards a predictable goal.”
As professional philosopher André Ariew in “” (pg. 8) explains:
“Insofar as Aristotle’s teleology pertains to explanations of natural items, it is misleading to cast off Aristotle’s teleology as reading purposive behavior into natural events. This perception of Aristotle’s teleology is the result of conflating Aristotle’s naturalistic teleology with Plato’s … Plato’s natural teleology is and Aristotle’s is not creationist, anthropomorphic, and externally evaluative … Aristotle’s natural teleology is and Plato’s is not naturalistic, immanent, and functional.”
Ariew (pgs. 8-9) goes on to identify two distinct types of teleology in Aristotle’s writings:
“Teleological explanation in Aristotle pertains broadly to goal-directed actions or behavior. Aristotle invokes teleology when an event or action pertains to goals … [W]e can distinguish two distinct conceptions of teleology in Aristotle’s writings and at least two sub-categories:
I. Agency-centered teleology:
i. Behavioral. Activities undertaken for the sake of something, which may be either a state or further action.
ii. Artifactual. Activities undertaken for the sake of producing an object of a certain sort (artificial).
II. Teleology pertaining to natural organisms.
i. Formal. Biological developmental processes that occur for the sake of self-preservation or preservation of the species (form).
ii. Functional. Parts of organisms that are present for the sake of the organism possessing them.
I and II are distinct notions of teleology … Agent-specific teleology (I) is purposive, rational, and intentional, and represents external evaluation. The goal is the object of an agent’s desire or choice … Teleology pertaining to natural organisms is distinct: non-purposive (though seemingly so), non-rational, non-intentional, and immanent — that is, an inner principle of change. The goal is not an object of any agent’s desire.”
This article makes the argument that agency-specific teleology (I) does not exist at the basic, irreducible level in naturalism, but does not argue that naturalism necessarily excludes Aristotle’s teleology pertaining to natural organisms (II). The agency-specific notion of teleology (I) also relates to Carrier’s discussion of irreducible mental states, since I am specifically referring to the sort of teleologies exhibited by minds and agents.