The universe is natural, with nothing beyond it

Atheists and secular humanists affirm that the universe is natural, with nothing beyond it, that reality consists of the physical, natural world, without a supernatural realm, deities, or transcendent order standing outside or above nature. On this view, philosophical naturalism, everything that exists is part of the single natural order and is in principle open to investigation and explanation by reason and science. There is no appeal to a creator beyond the cosmos or to spiritual substances distinct from the natural world.

Philosophers distinguish methodological naturalism, the working assumption that scientific inquiry should explain phenomena by natural causes without invoking the supernatural, from metaphysical or philosophical naturalism, the further and stronger claim that nothing supernatural actually exists; the philosopher Barbara Forrest's work on this distinction was cited at length in the 2005 United States federal court case Kitzmiller v. Dover Area School District, which examined whether intelligent design theory improperly imported supernatural causation into science education, and the case's reasoning has since been widely referenced in discussions of where naturalism as a scientific method ends and naturalism as a metaphysical worldview begins.

Naturalism is a foundational commitment of both atheism and secular humanism, underlying their further affirmations that this life is the only life and that meaning and morality are human responsibilities. It stands opposed to the theistic and supernaturalist frameworks of the religious traditions, and it aligns with the conviction that belief should follow evidence, since naturalists hold that no reliable evidence points beyond the natural world. The affirmation is presented here as the settled view of those who hold it.

Philosophical naturalism has ancient roots, notably in Epicurus and in the Roman poet Lucretius's De Rerum Natura, On the Nature of Things, which argued at length that the world operates by natural causes without divine intervention and that fear of the gods and of an afterlife causes needless human suffering; the position was revived and given a more explicitly scientific grounding in the twentieth century, notably by the philosopher W. V. O. Quine, whose account of naturalized epistemology argued that philosophy itself should be continuous with, rather than prior to or independent of, the findings of natural science, an influential modern statement of the naturalist position atheists and secular humanists generally draw upon.