Metaphysical naturalism, also known as ontological naturalism, philosophical naturalism, and antisupernaturalism, is a philosophical worldview that posits the universe contains only natural elements and principles, operating according to the laws of physics, and can be understood through science and philosophy. It stands in opposition to the supernatural and religious explanations. Its definition, as per Steven Schafersman, a geologist and president of Texas Citizens for Science, asserts that nature encompasses all that exists, includes only natural elements, and operates by the laws of physics. Supernatural phenomena do not exist in this worldview. Carl Sagan summarized this philosophy stating, "The Cosmos is all that is or ever was or ever will be."
The term "naturalism" has seen usage shift over time. Modern philosophers mostly accept naturalism as described by the early 20th-century American philosophers John Dewey, Ernest Nagel, Sidney Hook, and Roy Wood Sellars. According to philosopher Alvin Plantinga, a critic of naturalism, it functions cognitively similar to a religion in providing a set of answers to deep human questions.
Metaphysical naturalism, an approach to metaphysics, should not be confused with methodological naturalism, which is the foundation for the scientific method. The latter only requires empirical evidence and does not necessarily assert the ultimate truth of naturalism.
The philosophical basis of science, as per Kate and Vitaly (2000), rests on certain philosophical assumptions, including the objective and consistent nature of reality, the human capacity to accurately perceive reality, and the existence of rational explanations for elements of the real world.
The controversies around naturalism involve its perceived anthropic distinctions, the uniformity principle, and assumptions regarding spatial and temporal invariance. Other associated beliefs within metaphysical naturalism include the natural emergence of the universe, Earth, and life forms, the rejection of supernatural entities like souls or spirits, the idea of the mind as a natural phenomenon, and the utility of intelligence and reason.
The history of naturalism dates back to ancient and medieval philosophy, with the concept forming the foundation of several schools of Hinduism and Western philosophy. Western metaphysical naturalism originated in ancient Greek philosophy, with the earliest pre-Socratic philosophers and atomists investigating natural causes and excluding any role for gods in the creation or operation of the world.
In the history of philosophy, metaphysical naturalism, the belief that nature is all that exists and there are no supernatural entities, has had a varied and complex relationship with human thought. This philosophy contrasts with Plato's world of eternal and unchanging Forms, where matter is an imperfect representation created by a divine Artisan.
Aristotle, originally a follower of Plato, attempted to find a middle way between divine creation and random forces. He proposed that although there exists a divine being, the Prime Mover, it does not play a role in arranging the Universe. However, Aristotle didn't succumb to a pure mechanism of random forces, rather relying on the notion of Nature, or phusis.
With the rise of Christianity and Islam, metaphysical naturalism was generally abandoned by intellectuals, making little appearance in medieval philosophy. During the early modern era, philosophers like Benedict Spinoza, David Hume, and proponents of French materialism such as Denis Diderot and Baron d'Holbach brought naturalism back into philosophical discourse. During this period, materialism became the dominant category of metaphysical naturalism.
In the late modern era, philosophers Friedrich Wilhelm Joseph von Schelling and Georg Wilhelm Friedrich Hegel developed Naturphilosophie, an attempt to comprehend nature in its totality. This period also saw the rise of German materialism and the Marxist theory of dialectical materialism, both forms of naturalism.
Contemporary philosophy brought further advancements and revisions of naturalistic doctrines. Important developments in physics, mathematical logic, philosophy of language, philosophy of mind, ethics, and metaphysics led to the rise of new naturalistic philosophies such as physicalism, eliminative materialism, causal theories of reference, and ethical naturalism, among others. Currently, metaphysical naturalism is widely embraced, especially in the natural sciences and analytic philosophical communities.
David Papineau, a contemporary philosopher, states that the rise of naturalism is a consequence of scientific evidence accumulated during the 20th century for the "causal closure of the physical", the idea that all physical effects can be accounted for by physical causes. This led to the development of ontological naturalism, which asserts that any state that has physical effects must itself be physical.
There are arguments for and against metaphysical naturalism. Supporters argue that all known mental activity has a physical basis, thus supernatural minds likely do not exist. They also argue that cognitive biases are better explained by natural causes than as the work of God. However, critics argue that if all our thoughts are the effect of a physical cause, then we have no reason to assume that they are the result of a reasonable ground. Additionally, Alvin Plantinga, a philosopher, argues that evolution could not have produced humans with reliable true beliefs unless guided by God, thus disputing the naturalistic paradigm.