Metaphysical naturalism asserts that reality consists exclusively of natural entities and processes governed by natural laws, with no supernatural realm, beings, or interventions. This position claims that the natural world studied by the sciences exhausts what exists, making naturalistic explanations not merely methodologically preferable but metaphysically complete. The argument typically proceeds by asserting that (1) scientific methods provide our most reliable access to reality, (2) these methods have discovered only natural entities and laws, (3) there is no credible evidence for supernatural entities, and (4) therefore we should conclude that only the natural exists. This constitutes both an ontological thesis about what exists and an epistemological thesis about how we can know what exists.
The position emerged distinctly in the Enlightenment with figures like Baron d'Holbach's System of Nature (1770) and La Mettrie's Man a Machine (1748), though ancient precedents exist in Epicurus and Lucretius. Modern defenders include W.V.O. Quine's Naturalized Epistemology (1969), David Armstrong's A Materialist Theory of the Mind (1968), and Paul Churchland's Scientific Realism and the Plasticity of Mind (1979). Contemporary advocates include Alex Rosenberg's The Atheist's Guide to Reality (2011), Sean Carroll's The Big Picture (2016), and Graham Oppy's Naturalism and Religion (2018). The position gained prominence through logical positivism and remains influential in philosophy of mind and science.
Theistic philosophers argue that metaphysical naturalism cannot account for consciousness, as developed in David Chalmers' "hard problem" and Thomas Nagel's Mind and Cosmos (2012). They contend that abstract objects like numbers and logical laws cannot be naturalized, as argued by Alvin Plantinga's Where the Conflict Really Lies (2011). The fine-tuning of physical constants suggests design beyond natural processes, while moral realism seems to require non-natural foundations, as J.P. Moreland argues in The Recalcitrant Imago Dei (2009). Naturalists respond by developing physicalist theories of consciousness (Patricia Churchland), nominalist accounts of abstracta (Hartry Field), multiverse explanations for fine-tuning (Max Tegmark), and evolutionary or constructivist metaethics (Michael Ruse, Sharon Street).
Metaphysical naturalism differs from methodological naturalism, which merely adopts natural explanations as a research strategy without ontological commitment. Unlike physicalism, it need not reduce everything to physics, allowing for emergent properties and special sciences. It goes beyond causal closure (which only denies supernatural intervention in natural processes) by denying supernatural existence entirely. While reductionism seeks to explain higher-level phenomena through lower levels, metaphysical naturalism is compatible with non-reductive approaches that still deny the supernatural.