Alvin Plantinga has argued that Naturalism is stronger than just plain Atheism, and has characterized it as a sort of “Atheism+”. He suggests that it is possible for somebody to be an atheist without being a Naturalist, but not possible to be a Naturalist without being an Atheist. Something has always dissatisfied me with this, precisely because his definition of Naturalism goes something like this: “the belief that there is no such being as God, nor anything like God”. Now, not only is the “not anything like God” vague, but the Atheistic Naturalist will not likely see how that isn’t just a definition of Atheism in general. Of course, Plantinga probably means to exclude Buddhists, many of whom (the Mahayana school) believe in the Bodhisattvas (a sort of Buddhist equivalent to the Catholic doctrine of the communion of the saints). That could reasonably qualify as something “like God”, but it remains too vague for my liking. I don’t see why a Naturalist couldn’t believe in the soul consistently (not only consistent with Atheism, but consistent with forms of Naturalism) – and yet it isn’t clear that the idea of some immaterial and metaphysically simple substance isn’t also “like God”.
Perhaps a better definition is as follows: Naturalism is systematic Atheism. That is, a world view qualifies as Naturalistic just in case it takes, as its starting point, Atheism, and philosophically works out the paradoxes of the world from that perspective. Atheism thus acts as the cornerstone, the foundation, of Naturalism, and Naturalism stands or falls, it seems, with Atheism. Thus, ‘Naturalism’ designates a rather large and broad camp, but it will not include other kinds of atheists, such as Buddhists or Mormons. I suggest this definition works better.