First, I'll define naturalism as I see it. It's the concept that everything in the universe is purely a function of matter/energy and the fundamental forces of nature. Nothing supernatural exists. Which would be an entity that is claimed to be outside of, or beyond the natural realm. But has the ability to reach into the natural world and affect events therein.
I know I can't prove this with absolute metaphysical certainty. Naturalism, like god-belief, requires faith. In my case, that faith is a result of reasoning. Inductive reasoning to be specific. Our brains evolved to seek explanations. When one wasn't obvious, we created gods, spirits, or other supernatural entities to account for what we didn't understand. Thousands of such gods, in all cultures, have been created to explain things like weather, diseases, tidal action, floods, earthquakes, the change of the seasons, the motion of the sun, moon, and stars, and all other events that seemed inexplicable. But as our knowledge has increased, we know that all of these are perfectly natural phenomena. In all of history, a god, or supernatural force has never been a valid explanation for anything. So--by simple a posteriori logic--why should I believe any kind of supernatural power or god is responsible for all the things we still don't understand?
BTW: I'm certain that we never will be able to explain everything. The more we learn, the more questions will arise. But that's OK. That's what makes seeking to understand nature and nature's mechanisms exciting. The thrill is in the journey, not the destination.
