Naturalism, in essence, says that nature and matter are all that exist. For Sire, naturalism takes us one step further away from Christian theism. Historically, naturalism was the result of a gradual reduction of the idea of God. First, in deism, God’s personality and sustaining providence are denied; in naturalism, the existence of God is denied altogether.

Although none of them denied God’s existence—indeed, all professed theism—such thinkers as René Descartes, John Locke, and Julien de La Mettrie set the stage for naturalism. Descartes conceived of reality as divided between “matter” and “mind.” Naturalists went one step further by declaring that mind itself was merely a “subcategory of mechanistic matter” (56).

Thus, for naturalism the “prime reality is matter; matter exists eternally and is all there is. God does not exist” (57). This makes naturalism essentially a form of materialism. Since nothing can come of nothing, something always had to have existed. Naturalists say that this something was matter, not God or mind.

According to naturalism, the universe is a closed system in which things happen according to unchangeable laws of cause and effect. Nothing from outside the system of nature can change these laws for the simple reason that there is nothing outside the system of nature; nature (consisting of matter acting according to inexorable laws) is all that exists.