“Naturalism can intuitively be separated into a [metaphysical] and a methodological component.” Metaphysical here refers to the philosophical study of the nature of reality. 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.

In contrast, assuming naturalism in working methods, without necessarily considering naturalism as an absolute truth with philosophical entailments, is called methodological naturalism. The subject matter here is a philosophy of acquiring knowledge.

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.

As a cosmological argument for naturalism, Carl Sagan stated:

[Elegance] goes directly to the question of how the laws of nature are constructed. Nobody knows the answer to that. Nobody! It’s a perfectly legitimate hypothesis, in my view, to say that some extremely elegant creator made those laws. But I think if you go down that road, you must have the courage to ask the next question, which is: Where did that creator come from? And where did his, her, or its elegance come from? And if you say it was always there, then why not say that the laws of nature were always there and save a step?

— Carl Sagan, Conversations with Carl Sagan