Posted: Apr 16, 2013 8:03 pm
"In metaphysics naturalism is perhaps most obviously akin to materialism, but it does not have to be materialistic. What it insists on is that the world of nature should form a single sphere without incursions from outside by souls or spirits, divine or human, and without having to accommodate strange entities like non-natural values or substantive abstract universals. But it need not reject the phenomena of consciousness, nor even identify them somehow with material phenomena, as the materialist must, provided they can be studied via the science of psychology, which can itself be integrated into the other sciences. One naturalist in fact, Hume, was rather ambivalent about whether there was really a material world at all, except in so far as it was constructed out of our experiences, or impressions and ideas, as he called them. The important thing for the naturalist in the metaphysical sphere is that the world should be a unity in the sense of being amenable to a unified study which can be called the study of nature..."
But we can only integrate psychology into the other sciences because we assume (if we are metaphysical naturalists) that psychology depends on the physical reality of brains and people. And science depends on physical people doing experiments in physical laboratories with physical measuring equipment. If the unifying principle of naturalism was non-physical wouldn't we have to start calling all these stereotypical physical objects non-physical too? Wouldn't that just be an empty exercise in relabelling?
Physical laws are the unifying principle that constitutes naturalism. Or, to turn it round, whatever unifying principle there was we would call it physical because that what we mean by physical. Even Hume might agree with that.