Methodological naturalism is the principle that scientific inquiry should proceed by seeking natural causes and explanations for phenomena, without invoking supernatural entities or divine intervention. This approach claims that while science cannot definitively prove or disprove the existence of God, the scientific method must operate as if only natural causes exist, treating the universe as a causally closed system amenable to empirical investigation. The argument against theism emerges when this methodological stance is taken to imply that supernatural explanations are either unnecessary or inadmissible in understanding reality, suggesting that the success of naturalistic science undermines warrant for theistic belief.
The principle has roots in the scientific revolution, with Francis Bacon's Novum Organum (1620) advocating empirical method over theological speculation. Modern formulations emerged through figures like Charles Lyell's Principles of Geology (1830-1833), which established uniformitarian methodology, and Thomas Huxley's advocacy for scientific naturalism in Evidence as to Man's Place in Nature (1863). Contemporary defenders include Michael Ruse in The Science-Religion Dialogue (2010), Robert Pennock in Tower of Babel (1999), and Barbara Forrest in Creationism's Trojan Horse (2004). Philosophers of science like Paul Kurtz in The Transcendental Temptation (1986) and Maarten Boudry in his work on demarcation criteria have refined arguments about why methodological naturalism constitutes best practice in scientific inquiry.
Theistic philosophers respond that methodological naturalism artificially constrains inquiry and may blind science to evidence of design or divine action. Alvin Plantinga in Where the Conflict Really Lies (2011) argues that methodological naturalism lacks philosophical justification and amounts to question-begging against theism. Del Ratzsch in Nature, Design, and Science (2001) contends that ruling out supernatural explanations a priori is dogmatic rather than empirical. Proponents counter that methodological naturalism's success in generating reliable knowledge vindicates its approach, that supernatural explanations have historically impeded scientific progress, and that allowing non-natural causes would make science unfalsifiable and destroy its predictive power.
Unlike metaphysical naturalism, which makes the ontological claim that only natural entities exist, methodological naturalism is ostensibly neutral about ultimate reality while prescribing investigative method. It differs from causal closure, which specifically asserts that physical effects have only physical causes, and from reductionism, which claims higher-level phenomena reduce to lower-level physical processes. While physicalism makes claims about the fundamental nature of reality, methodological naturalism concerns only the proper approach to scientific investigation.