rationalskepticism.org

"Naturalism. In general the view that everything is natural, i.e. that everything there is belongs to the world of nature, and so can be studied by the methods appropriate for studying that world, and the apparent exceptions can be somehow explained away. ...The more general application is to philosophy as a whole, and again involves both the objects studied and the methods used in studying them, i.e. both metaphysics and epistemology. 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, though it may not always be easy to say what counts as a sufficient degree of unification. Obviously there are different sciences, which to some extent employ different methods as well as studying different subject-matters. What seems to be needed is that they should form a continuous chain, and all be subject to certain general requirements regarded as necessary for a science as such, like producing results which are amenable to empirical testing. Whatever entities such sciences come up with must then be allowed into the naturalistic framework, and these will include ‘theoretical’ entities which cannot be directly observed, but whose existence is postulated to explain various phenomena, such as the electrons of physics, whether this existence is taken to be real or only ‘logically constructed’ in the way in which the average man is logically constructed out of ordinary men."

("Naturalism." In The Oxford Companion to Philosophy, 2nd ed., edited by Ted Honderich, 640-642. Oxford: Oxford University Press, 2005. pp. 640-1)