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Naturalism is the philosophical theory that treats science as our most reliable source of knowledge and scientific method as the most effective route to knowledge. In a recent essay for The Stone, Timothy Williamson correctly reports that naturalism is popular in philosophy. In fact it is now a dominant approach in several areas of philosophy — ethics, epistemology, the philosophy of mind, philosophy of science and, most of in all, metaphysics, the study of the basic constituents of reality. Metaphysics is important: if it turns out that reality contains only the kinds of things that hard science recognizes, the implications will be grave for what we value in human experience.

Naturalism is itself a theory with a research agenda of unsolved problems. But naturalists’ confidence that it can solve them shouldn’t be mistaken for “dogmatism,” nor can its successes be written off as “slick packaging,” two terms Professor Williamson used in his essay to describe why he rejects naturalism.

Before taking up Professor Williamson’s challenges to naturalism, it’s worth identifying some of this success in applying science to the solution of philosophical problems, some of which even have pay-offs for science. Perhaps the most notable thing about naturalism is the way its philosophers have employed Darwin’s theory of natural selection to tame purpose. In 1784 Kant wrote, “There will never be a Newton for the blade of grass.” What he meant was that physical science could never explain anything with a purpose, whether it be human thought or a flower’s bending toward the sun. That would have made everything special about living things — and especially us — safe from a purely scientific understanding. It would have kept questions about humanity the preserve of religion, mythmaking and the humanities.

Only 25 years or so later, the Newton of the blade of grass was born to the Darwin family in Shropshire, England. “On the Origin of Species” revealed how physical processes alone produce the illusion of design. Random variation and natural selection are the purely physical source of the beautiful means/ends economy of nature that fools us into seeking its designer. Naturalists have applied this insight to reveal the biological nature of human emotion, perception and cognition, language, moral value, social bonds and political institutions. Naturalistic philosophy has returned the favor, helping psychology, evolutionary anthropology and biology solve their problems by greater conceptual clarity about function, adaptation, Darwinian fitness and individual-versus-group selection.

While dealing with puzzles that vexed philosophy as far back as Plato, naturalism has also come to grips with the very challenges Professor Williamson lays out: physics may be our best take on the nature of reality, but important parts of physics are not just “abstract,” as he says. Quantum mechanics is more than abstract. It’s weird. Since naturalistic philosophers take science seriously as the best description of reality, they accept the responsibility of making sense of quantum physics. Until we succeed, naturalists won’t be any more satisfied than Professor Williamson that we know what the natural world is. But 400 years of scientific success in prediction, control and technology shows that physics has made a good start. We should be confident that it will do better than any other approach at getting things right.

Naturalists recognize that science is fallible. Its self-correction, its continual increase in breadth and accuracy, give naturalists confidence in the resources they borrow from physics, chemistry and biology. The second law of thermodynamics, the periodic table, and the principles of natural selection are unlikely to be threatened by future science. Philosophy can therefore rely on them to answer many of its questions without fear of being overtaken by events.

“Why can’t there be things only discoverable by non-scientific means, or not discoverable at all?” Professor Williamson asked in his essay. His question may be rhetorical, but the naturalist has an answer to it: nothing that revelation, inspiration or other non-scientific means ever claimed to discover has yet to withstand the test of knowledge that scientific findings attain. What are those tests of knowledge? They are the experimental/observational methods all the natural sciences share, the social sciences increasingly adopt, and that naturalists devote themselves to making explicit. You can reject naturalists’ epistemology, or treat it as question begging, but you can’t accuse them of not having one.

As Professor Williamson notes, naturalism’s greatest challenge “is to find a place for mathematics.” The way it faces the challenge reveals just how undogmatic naturalism really is. It would be easy to turn one’s back on the problems mathematics presents (What are numbers? How can we have the certainty about them that math reveals?). One excuse to turn our backs is that mathematicians and scientists don’t care much about these problems; another is that no one has ever provided a satisfactory answer to these questions, so no other philosophy can be preferred to naturalism on this basis. But naturalism has invested a huge amount of ingenuity, even genius, seeking scientifically responsible answers to these hardest of questions. Not with much success as yet by our own standards, one must admit. But that is the nature of science.

Naturalism takes the problem of mathematics seriously since science cannot do with out it. So naturalism can’t either. But what about other items on Professor Williamson’s list of disciplines it would be hard to count as science: history, literary theory? Can science and naturalistic philosophy do without them? This is a different question from whether people, as consumers of human narratives and enjoyers of literature, can do without them. The question naturalism faces is whether disciplines like literary theory provide real understanding?

Naturalism faces these questions because it won’t uncritically buy into Professor Williamson’s “default assumption … that the practitioners of a well-established discipline know what they are doing, and use the … methods most appropriate for answering its questions.” If semiotics, existentialism, hermeneutics, formalism, structuralism, post-structuralism, deconstruction and post-modernism transparently flout science’s standards of objectivity, or if they seek arbitrarily to limit the reach of scientific methods, then naturalism can’t take them seriously as knowledge.

That doesn’t mean anyone should stop doing literary criticism any more than foregoing fiction. Naturalism treats both as fun, but neither as knowledge

What naturalists really fear is not becoming dogmatic or giving up the scientific spirit. It’s the threat that the science will end up showing that much of what we cherish as meaningful in human life is illusory.

Alex Rosenberg is the R. Taylor Cole Professor and philosophy department chair at Duke University. He is the author of 12 books in the philosophy of biology and economics. W.W. Norton will publish his latest book, “The Atheist’s Guide to Reality,” in October.