Naturalism is the “idea or belief that only natural (as opposed to supernatural or spiritual) laws and forces operate in the world.” – Oxford English Dictionary Online
In philosophy, naturalism is the “idea or belief that only natural (as opposed to supernatural or spiritual) laws and forces operate in the world.” Adherents of naturalism (i.e., naturalists) assert that natural laws are the rules that govern the structure and behavior of the natural universe, that the changing universe at every stage is a product of these laws.
“Naturalism can intuitively be separated into an ontological and a methodological component.” “Ontological” refers to the philosophical study of the nature of reality. Some philosophers equate naturalism with materialism. For example, philosopher Paul Kurtz argues that nature is best accounted for by reference to material principles. These principles include mass, energy, and other physical and chemical properties accepted by the scientific community. Further, this sense of naturalism holds that spirits, deities, and ghosts are not real and that there is no “purpose” in nature. Such an absolute belief in naturalism is commonly referred to as metaphysical naturalism.
Assuming naturalism in working methods is the current paradigm, without the unfounded consideration of naturalism as an absolute truth with philosophical entailment, called methodological naturalism. The subject matter here is a philosophy of acquiring knowledge based on an assumed paradigm.
With the exception of pantheists—who believe that Nature and God are one and the same thing—theists challenge the idea that nature contains all of reality. According to some theists, natural laws may be viewed as so-called secondary causes of god(s).
In the 20th century, Willard Van Orman Quine, George Santayana, and other philosophers argued that the success of naturalism in science meant that scientific methods should also be used in philosophy. Science and philosophy are said to form a continuum, according to this view.
 "CATHOLIC ENCYCLOPEDIA: Naturalism". 21 November 2009. Retrieved 6 March 2012. Naturalism is not so much a special system as a point of view or tendency common to a number of philosophical and religious systems; not so much a well-defined set of positive and negative doctrines as an attitude or spirit pervading and influencing many doctrines. As the name implies, this tendency consists essentially in looking upon nature as the one original and fundamental source of all that exists, and in attempting to explain everything in terms of nature. Either the limits of nature are also the limits of existing reality, or at least the first cause, if its existence is found necessary, has nothing to do with the working of natural agencies. All events, therefore, find their adequate explanation within nature itself. But, as the terms nature and natural are themselves used in more than one sense, the term naturalism is also far from having one fixed meaning.  Papineau, David (22 February 2007). "Naturalism". Stanford Encyclopedia of Philosophy.  Kurtz, Paul (Spring 1998). "Darwin Re-Crucified: Why Are So Many Afraid of Naturalism?". Free Inquiry. 18 (2).  Schafersman, Steven D. (1996). "Naturalism is Today An Essential Part of Science". Methodological naturalism is the adoption or assumption of naturalism in scientific belief and practice without really believing in naturalism.  Jonathan Barnes's introduction to Early Greek Philosophy (Penguin)