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Naturalism commonly refers to the viewpoint that laws of nature (as opposed to supernatural ones) operate in the universe, and that nothing exists beyond the natural universe or, if it does, it does not affect the natural universe. Adherents of naturalism (naturalists) assert that natural laws are the rules that govern the structure and behavior of the natural universe, that the universe is a product of these laws.
Philosopher Paul Kurtz argues that nature is best accounted for by reference to material principles. These principles include mass, energy, and other physical and chemical properties accepted by the scientific community. Further, this sense of naturalism holds that spirits, deities, and ghosts are not real and that there is no "purpose" in nature. Such an absolute belief in naturalism is usually referred to as metaphysical naturalism (or philosophical naturalism). In contrast, assuming naturalism in working methods, without necessarily considering naturalism as an absolute truth with philosophical entailments, is called methodological naturalism.
Theists challenge the idea that nature is all there is. They believe in one or more gods that created nature. Natural laws have a place in their theology; they describe the effects of so-called secondary causes (see History section, below). But, natural laws do not define nor limit deities as primary causes.
In the 20th century, W.V. Quine, George Santayana, and other philosophers argued that the success of naturalism in science meant that scientific methods should also be used in philosophy. Science and philosophy are said to form a continuum, according to this view.
The ideas and assumptions of philosophical naturalism were first seen in the works of the Ionian pre-Socratic philosophers. One such was Thales, considered to be the father of science, as he was the first to give explanations of natural events without the use of supernatural causes. These early philosophers subscribed to principles of empirical investigation that strikingly anticipate naturalism.
During the Enlightenment, a number of philosophers including Francis Bacon and Voltaire outlined the philosophical justifications for removing appeal to supernatural forces from investigation of the natural world. Subsequent scientific revolutions would offer modes of explanation not inherently theistic for biology, geology, physics, and other natural sciences.
According to Steven Schafersman, president of Texas Citizens for Science, an advocacy group opposing creationism in public schools, the progressive adoption of methodological naturalism—and later of metaphysical naturalism—followed the advances of science and the increase of its explanatory power. These advances also caused the diffusion of positions associated with metaphysical naturalism, such as existentialism.
The current usage of the term naturalism "derives from debates in America in the first half of the last century. The self-proclaimed 'naturalists' from that period included John Dewey, Ernest Nagel, Sidney Hook and Roy Wood Sellars." For them nature is the only reality. There is no such thing as 'supernatural'. The scientific method is to be used to investigate all reality, including the human spirit: "So understood, 'naturalism' is not a particularly informative term… The great majority of contemporary philosophers would happily… reject 'supernatural' entities, and allow that science is a possible route (if not necessarily the only one) to important truths about the 'human spirit'."
The term "methodological naturalism" for this approach is much more recent. According to Ronald Numbers, it was coined in 1983 by Paul de Vries, a Wheaton College philosopher. De Vries distinguished between what he called "methodological naturalism," a disciplinary method that says nothing about God's existence, and "metaphysical naturalism," which "denies the existence of a transcendent God." The term "methodological naturalism" had been used in 1937 by Edgar S. Brightman in an article in The Philosophical Review as a contrast to "naturalism" in general, but there the idea was not really developed to its more recent distinctions.
Metaphysical naturalism, also called "ontological naturalism" and "philosophical naturalism", is a philosophical worldview and belief system that holds that there is nothing but natural elements, principles, and relations of the kind studied by the natural sciences, i.e., those required to understand our physical environment by mathematical modeling. Methodological naturalism, on the other hand, refers exclusively to the methodology of science, for which metaphysical naturalism provides only one possible ontological foundation.
Metaphysical naturalism holds that all properties related to consciousness and the mind are reducible to, or supervene upon, nature. Broadly, the corresponding theological perspective is religious naturalism or spiritual naturalism. More specifically, metaphysical naturalism rejects the supernatural concepts and explanations that are part of many religions.
Methodological naturalism is concerned not with claims about what exists but with methods of learning what is nature. It is strictly the idea that all scientific endeavors—all hypotheses and events—are to be explained and tested by reference to natural causes and events. The genesis of nature, e.g., by an act of God, is not addressed. This second sense of naturalism seeks only to provide a framework within which to conduct the scientific study of the laws of nature. Methodological naturalism is a way of acquiring knowledge. It is a distinct system of thought concerned with a cognitive approach to reality, and is thus a philosophy of knowledge. Studies by sociologist Elaine Ecklund suggest that religious scientists do in fact apply methodological naturalism. They report that their religious beliefs affect the way they think about the implications, often moral, of their work, but not the way they practice science.
In a series of articles and books from 1996 onwards, Robert T. Pennock wrote using the term methodological naturalism to clarify that the scientific method confines itself to natural explanations without assuming the existence or non-existence of the supernatural, and is not based on dogmatic metaphysical naturalism as claimed by creationists and proponents of intelligent design, in particular Phillip E. Johnson. Pennock's testimony as an expert witness at the Kitzmiller v. Dover Area School District trial was cited by the Judge in his Memorandum Opinion concluding that "Methodological naturalism is a "ground rule" of science today":
Robert T. Pennock contends that as supernatural agents and powers "are above and beyond the natural world and its agents and powers" and "are not constrained by natural laws", only logical impossibilities constrain what a supernatural agent could not do, and "If we could apply natural knowledge to understand supernatural powers, then, by definition, they would not be supernatural". As the supernatural is necessarily a mystery to us, it can provide no grounds on which to judge scientific models. "Experimentation requires observation and control of the variables.... But by definition we have no control over supernatural entities or forces." Science does not deal with meanings; the closed system of scientific reasoning cannot be used to define itself. Allowing science to appeal to untestable supernatural powers would make the scientist's task meaningless, undermine the discipline that allows science to make progress, and "would be as profoundly unsatisfying as the ancient Greek playwright's reliance upon the deus ex machina to extract his hero from a difficult predicament."
Naturalism of this sort says nothing about the existence or nonexistence of the supernatural, which by this definition is beyond natural testing. As a practical consideration, the rejection of supernatural explanations would merely be pragmatic, thus it would nonetheless be possible, for an ontological supernaturalist to espouse and practice methodological naturalism. For example, a scientist may believe in God, while practicing methodological naturalism in their scientific work. This position does not preclude knowledge that is somehow connected to the supernatural. Generally however, anything that can be scientifically examined and explained would not be supernatural, simply by definition.
Alvin Plantinga is a Professor of Philosophy now at Calvin College, a Christian, and a well-known critic of naturalism. He argues, in his evolutionary argument against naturalism, that the probability that evolution has produced humans with reliable true beliefs, is low or inscrutable, unless their evolution was guided, for example, by God. According to David Kahan of the University of Glasgow, in order to understand how beliefs are warranted, a justification must be found in the context of supernatural theism, as in Plantinga's epistemology. (See also supernormal stimuli).
Plantinga argues that together, naturalism and evolution provide an insurmountable "defeater for the belief that our cognitive faculties are reliable", i.e., a skeptical argument along the lines of Descartes' Evil demon or Brain in a vat.
Take philosophical naturalism to be the belief that there aren't any supernatural entities--no such person as God, for example, but also no other supernatural entities, and nothing at all like God. My claim was that naturalism and contemporary evolutionary theory are at serious odds with one another--and this despite the fact that the latter is ordinarily thought to be one of the main pillars supporting the edifice of the former. (Of course I am not attacking the theory of evolution, or anything in that neighborhood; I am instead attacking the conjunction of naturalism with the view that human beings have evolved in that way. I see no similar problems with the conjunction of theism and the idea that human beings have evolved in the way contemporary evolutionary science suggests.) More particularly, I argued that the conjunction of naturalism with the belief that we human beings have evolved in conformity with current evolutionary doctrine… is in a certain interesting way self-defeating or self-referentially incoherent.
— Alvin Plantinga, Naturalism Defeated?: Essays on Plantinga's Evolutionary Argument Against Naturalism, "Introduction"
What are the advantages and disadvantages of doing science in accord with methodological naturalism?...if you exclude the supernatural from science, then if the world or some phenomena within it are supernaturally caused — as most of the world's people believe — you won't be able to reach that truth scientifically. Observing methodological naturalism thus hamstrings science by precluding science from reaching what would be an enormously important truth about the world.
— Alvin Plantinga, Science & Theology News, "Whether ID is science isn't semantics"
W. V. Quine describes naturalism as the position that there is no higher tribunal for truth than natural science itself. There is no better method than the scientific method for judging the claims of science, and there is neither any need nor any place for a "first philosophy", such as (abstract) metaphysics or epistemology, that could stand behind and justify science or the scientific method.
Therefore, philosophy should feel free to make use of the findings of scientists in its own pursuit, while also feeling free to offer criticism when those claims are ungrounded, confused, or inconsistent. In this way philosophy becomes "continuous with" science. Naturalism is not a dogmatic belief that the modern view of science is entirely correct. Instead, it simply holds that science is the best way to explore the processes of the universe and that those processes are what modern science is striving to understand. However, this Quinean Replacement Naturalism finds relatively few supporters among philosophers.
Karl Popper equated naturalism with inductive theory of science. He rejected it based on his general critique of induction (see problem of induction), yet acknowledged its utility as means for inventing conjectures.
A naturalistic methodology (sometimes called an "inductive theory of science") has its value, no doubt.... I reject the naturalistic view: It is uncritical. Its upholders fail to notice that whenever they believe to have discovered a fact, they have only proposed a convention. Hence the convention is liable to turn into a dogma. This criticism of the naturalistic view applies not only to its criterion of meaning, but also to its idea of science, and consequently to its idea of empirical method.
Popper instead proposed that science should adopt a methodology based on falsifiability for demarcation, because no number of experiments can ever prove a theory, but a single experiment can contradict one. Popper holds that empirical theories are characterized by falsifiability.