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.