"a species of philosophical monism according to which whatever exists or happens is natural in the sense of being susceptible to explanation through methods which, although paradigmatically exemplified in the natural sciences, are continuous from domain to domain of objects and events...[thus, there cannot] exist any entities or events which lie, in principle, beyond the scope of scientific explanation" (Danto, 1967, p. 448); "the view that nature is all there is and all basic truths are truths of nature" (Audi, 1996, p. 372);
"the twofold view that (1) everything is composed of natural entities--those studied in the sciences--whose properties determine all the properties of things, persons included, ...abstract entities... like possibilities...and mathematical objects...and (2) acceptable methods of justification and explanation are commensurable, in some sense, with those in science" (Post, 1995, p. 517);
"the view that everything is natural, i.e. that everything there is belongs to the world of nature, and so can be studied by the methods appropriate for studying that world..." (Lacey, 1995, p. 604);
"the philosophical movement that "wishes to use the methods of science, evidence, and reason to understand nature and the place of human species within it"..."skeptical of the postulation of a transcendental realm beyond nature, or of the claim that nature can be understood without using the methods of reason and evidence"... and "the philosophical generalization of the methods and conclusions of the sciences" (Kurtz, 1990, p. 7, 12).
In my own definition, a synthesis of those above, naturalism is the philosophy that maintains that (1) nature is all there is and whatever exists or happens is natural; (2) nature (the universe or cosmos) consists only of natural elements, that is, of spatiotemporal material elements--matter and energy--and non-material elements--mind, ideas, values, logical relationships, etc.--that are either associated with the human brain or exist independently of the brain and are therefore somehow immanent in the structure of the universe; (3) nature works by natural processes that follow natural laws and can, in principle, be explained and understood by science and philosophy; and (4) the supernatural does not exist, i.e., only nature is real, therefore, supernature is non-real. Naturalism is therefore a metaphysical philosophy opposed primarily by supernaturalism.
Naturalism is, noncontroversally, a subset of metaphysical realism. Naturalism is not an ethical system, although a variety--pragmatic naturalism, a synthesis of pragmatism and naturalism--does develop ethical positions. Philosophical naturalism is also the key part of naturalistic...