“Naturalism asserts, first of all, that the primary constituents of reality are material entities. By this I do not mean that only material entities exist; I do not wish to deny the reality-the real existence-of such things as hopes, plans, behavior, language, logical inferences, and so on. What I am asserting, however, is that anything that is real is in the last analysis explicable as a material entity or as a form or function or action, etc., of a material entity…If the theoretical goal of science-an absolutely exhaustive knowledge of the natural world-were to be achieved, there would remain no reality of any other kind about which we might still be ignorant. The “ultimate realities,” according to naturalism, are not the alleged objects of the inquiries of theologians: they are the entities that are the objects of investigation by chemists, physicists, and other scientists. The put the matter very simply: materialism is true.
Naturalism asserts, secondly, that what happens in the world is theoretically explicable without residue in terms of the internal structure and the external relations of these material entities. The world is-to use a very inadequate metaphor-like a gigantic machine who parts are so numerous and whose processes are so complex that we have thus far been able to achieve only a very partial and fragmentary understanding of how it works. In principle, however, everything that occurs is ultimately explicable in terms of the properties and relations of the participles of which matter is composed. Once again the point may be stated simply: determinism is true.
It follows from what I have said that the categories of space and time are categories of great importance for naturalism-are, in fact, ontological categories. If you cannot locate something in space and time, or if you cannot understand it as a form or function of some entity or entities located in space and time, then you simply cannot say anything intelligible about it. To be is to be some place, some time.
Naturalism, therefore, denies the existence of any real entities corresponding to such concepts as God, angel, devil, spirit, or soul. The reason for denying the existence of such entities is that they are asserted by those who affirm their existence to be non-material entities that are nonetheless the subjects of activities of various kinds-they allegedly do such things as think, decide, regret, suffer, etc. But from the point of view of naturalism, any activity must ultimately be understood as an occurrence within space and time of processes involving material entities. Since the above concepts cannot be understood in this way, naturalism cannot but regard them as bogus concepts, concepts that claim to denote some real entities but in fact denote nothing at all.”
(Halverson, A Concise Introduction to Philosophy, 451-52)