Put most succinctly, metaphysical naturalism affirms that the natural world is the only real one, and that the human race is not separate from it, but belongs to it as a part. The term naturalism refers also to an aesthetic style in literature, drama, and painting, and in ethics, to the theory that the full meaning of value concepts such as good and evil can be spelled out using only terms from a natural, or factual vocabulary. These are not of concern here. What follows is a discussion of naturalism in metaphysics and epistemology.

Everyone has a rough working notion of what can happen in the course of nature, and is familiar with the idea that perhaps a transcendent or supernatural realm lies beyond nature, another world that may occasionally make contact with the everyday world by, for example, miraculous interventions. Yet the distinction between what is natural...